Use local variables in _form.html.erb generated by scaffold.

Conflicts:
	railties/CHANGELOG.md

Introduce explicit way of halting callback chains by throwing :abort. Deprecate current implicit behavior of halting callback chains by returning `false` in apps ported to Rails 5.0. Completely remove that behavior in brand new Rails 5.0 apps.

Conflicts:
	railties/CHANGELOG.md

Remove the tmp/sessions folder and its clear task

Commit 1aea4704 introduced this directory but this was at a time when the
default way to store sessions was on the file system under the tmp
directory.

Let's remove references to it from the documentation as well.

[Robin Dupret & yui-knk]

Some guides improvements

Commit 65a2977c added a `pre-wrap` style for white spaces on `.note`
paragraphs. However, this is first inconsistent as other notes like
warnings don't have this style applied. Furthermore, it seems to be
unneeded for mobile devices.

Also revert changes made in #18147 since they aren't needed anymore.

Cross-refs #18138.

[ci skip]

To highlight ERB snippets, both `erb` and `html+erb` are used as a
language passed to the code block. However, only the former will be
correctly highlighted.

Generated fixtures won't use parent_id when generated with
parent:references

Fix #18301

Assert that 2 letter tlds with 3 letter domain names work when option specified.

Fully support datetime values in AR::Type::DateTime#type_cast_for_database

Unsafe default translations should not be marked html_safe

This stems from [a comment](rails#17227 (comment)) by @dhh.
In summary:

* New Rails 5.0 apps will not accept `return false` as a way to halt callback chains, and will not display a deprecation warning.
* Existing apps ported to Rails 5.0 will still accept `return false` as a way to halt callback chains, albeit with a deprecation warning.

For this purpose, this commit introduces a Rails configuration option:

```ruby
config.active_support.halt_callback_chains_on_return_false
```

For new Rails 5.0 apps, this option will be set to `false` by a new initializer
`config/initializers/callback_terminator.rb`:

```ruby
Rails.application.config.active_support.halt_callback_chains_on_return_false = false
```

For existing apps ported to Rails 5.0, the initializers above will not exist.
Even running `rake rails:update` will not create this initializer.

Since the default value of `halt_callback_chains_on_return_false` is set to
`true`, these apps will still accept `return true` as a way to halt callback
chains, displaying a deprecation warning.

Developers will be able to switch to the new behavior (and stop the warning)
by manually adding the line above to their `config/application.rb`.

A gist with the suggested release notes to add to Rails 5.0 after this
commit is available at https://gist.github.com/claudiob/614c59409fb7d11f2931

Before this commit, returning `false` in an ActiveRecord `before_` callback
such as `before_create` would halt the callback chain.

After this commit, the behavior is deprecated: will still work until
the next release of Rails but will also display a deprecation warning.

The preferred way to halt a callback chain is to explicitly `throw(:abort)`.

Before this commit, returning `false` in an ActiveModel `before_` callback
such as `before_create` would halt the callback chain.

After this commit, the behavior is deprecated: will still work until
the next release of Rails but will also display a deprecation warning.

The preferred way to halt a callback chain is to explicitly `throw(:abort)`.

Before this commit, returning `false` in an ActiveModel validation
callback such as `before_validation` would halt the callback chain.

After this commit, the behavior is deprecated: will still work until
the next release of Rails but will also display a deprecation warning.

The preferred way to halt a callback chain is to explicitly `throw(:abort)`.

After this commit, returning `false` in a callback will display a deprecation
warning to make developers aware of the fact that they need to explicitly
`throw(:abort)` if their intention is to halt a callback chain.

This commit also patches two internal uses of AS::Callbacks (inside
ActiveRecord and ActionDispatch) which sometimes return `false` but whose
returned value is not meaningful for the purpose of execution.

In both cases, the returned value is set to `true`, which does not affect the
execution of the callbacks but prevents unrequested deprecation warnings from
showing up.

This commit changes arguments and default value of CallbackChain's :terminator
option.

After this commit, Chains of callbacks defined **without** an explicit
`:terminator` option will be halted as soon as a `before_` callback throws
`:abort`.

Chains of callbacks defined **with** a `:terminator` option will maintain their
existing behavior of halting as soon as a `before_` callback matches the
terminator's expectation. For instance, ActiveModel's callbacks will still
halt the chain when a `before_` callback returns `false`.

This commit modifies the code (but not the purpose) of a test that checks that

> initializers are executed after application configuration initializers

Currently the test hard-codes the *exact* initializers that are expected to
occur before a custom one. This can cause the test to fail even if the
expectation still passes.

This commit loosens the test by simply checking that, in the array of
initializers, the custom initializers (called `dummy_initializer` in the
example) is executed after the last occurrence of `load_config_initializers`.

Previously default translation keys that didn't end in `_html`, but came
after a missing key that ended in `_html` were being returned as
html_safe. Now they are not. Fixes #18257

Refactoring of add_constraints in AssociationScope

Fix TaggedLogging to allow loggers to be instantiated multiple times without having to share the stack of tags

`MissingSourceFile` was just an alias to `LoadError` and was not
being raised inside the framework.

replace use of MissingSourceFile with LoadError

Conflicts:
	activesupport/test/core_ext/load_error_test.rb

Also prevents the word "Model" from linking to the documentation
of ActiveModel::Model because that's not intended.

[ci skip]

Add missing nodoc's
Change `assoc_klass` argument name to `association_klass`
Change `prev_reflection` argument name to `previous_reflection`
Change `prev` to `previous_reflection` in `#get_chain`
Switch use of `refl` and `reflection` in `#get_chain` so main parameter
is not abbreviated.
Add missing space in `#add_constraints`

This moves the `#type_caster` from the `aliased_table_for` and into the
initialize of the `alias_tracker`.

`yield` instead of relying on checking if the reflection is equal to the
`chain_head`.

Instead of initializing an empty connection use the base table name
instead. Split up and refactor `#create` to be 2 methods `#create` and
`#create_with_joins`. Removes the need to update the count by 1 on
initialzing a JoinDependency.

This moves `alias_candidate` out of the `ReflectionProxy` and into the
`AbstractReflection` so it is shared by all reflections. Change
`alias_name` to a method and and remove assignment in `#get_chain`.

After the refactorings we're only using the connection and not the alias
tracker anymore. This builds on commit 18019.

Reuse the already available `@connection` to reduce the surface area of
the alias tracker's API. We can then remove the `attr_reader` because
the connection is already available.

This makes the `#alias_name` more functional.

Move `RuntimeReflection` and `PolymorphicReflect` into Reflection. This
allows the methods to inherit from `ThroughReflection` and DRY up the
methods by removing duplicates.

The linked list lets us use a loop in `#add_constraints` and completely
remove the need for indexing the iteration becasue we have access to the
next item in the chain.

By concatnating the `ReflectionProxy` with the `chain` we remove
the need for `#construct_tables` because the `chain` is now in the
correct order (order of the chain DOES matter).