Prevents double submission by making disable_with the default.

Default disable_with option will only be applied if user has not
specified her/his own disable_with option, whether that is in the
`data-disable-with` string form or the
`:data => { :disable_with => "Saving..." }` hash form. disable_with
will default to the value attribute.

A configuration option was added to opt out of this functionality if
the user so desires.
`config.action_view.automatically_disable_submit_tag = false`

so replace `Active Record` with `Active Model`

As per railties/lib/rails/test_unit/railtie.rb, where it sets it explicitly.

This can be confirmed by starting a new Rails console session in a new app and running this code:

Rails.application.config.app_generators.rails[:integration_tool]

I also beefed up the documentation slightly to be more explicit about what this configuration setting does.

[ci skip]

[Robin Dupret & Shunsuke Aida]

Also add a CHANGELOG entry for #18587

[ci skip]

This option has been removed in e6747d87

[ci skip]

Set `config.static_index` to serve a static directory index file not
named `index`. For example, to serve `main.html` instead of `index.html`
for directory requests, set `config.static_index` to `"main"`.

Fix the guide to state that Rails uses Minitest as the default test
framework.

Remove unnecessary mention to Test::Unit from the API docs
('constantize' and 'safe_constantize').

Explain config.active_support.test_order defaults
to :random in newly-generated applications thanks
to generated config/environments/test.rb, otherwise
default is :sorted (until Rails 5.0).

This is a squash of the following commits, from first to last:

-

Fix minor, random things I’ve come across lately that individually
did not seem worth making a PR for, so I saved them for one commit.

One common error is using “it’s” (which is an abbreviation of “it is”)
when the possessive “its” should be used for indicating possession.

-

Changes include the name of a test, so remove the `[skip ci]` (thanks @senny).

-

Line wrap the changes at 80 chars and add one more doc fix.

-

Add a missing line wrap in the Contributing to Ruby on Rails Guide.

-

Line wrap the `TIP` section in the Contributing to Ruby on Rails Guide as well.

Rendering the guide locally with `bundle exec rake guides:generate` did
not show any change in on-screen formatting after adding the line wrap.

The HTML generated is (extra line added to illustrate where the line
wrap takes place):

    <div class="info"><p>Please squash your commits into a single commit
when appropriate. This

    simplifies future cherry picks and also keeps the git log
clean.</p></div>

-

Squash commits.

Add `config.active_record.warn_on_records_fetched_greater_than` to the
Configuring Rails Guide.

Fixes db:structure:dump when using schema_search_path and PostgreSQL
extensions.

Closes #17157.

Mention that tags can also come from a Proc that accepts the request
object or something's to_s method

* Fix a few typos
* Wrap some lines around 80 chars
* Rephrase some statements

[ci skip]

Serving public files from multiple paths was removed in
afc82882

Deprecate `required` option in favor of `optional` for belongs_to.

* "rm docs for dependency_loading and disable_dependency_loading config"

It was added back in a71350ca by @tenderlove

See also #13142

removes docs for dependency_loading config option and disable_dependency_loading initializer because it was removed by commit a8bf1297

- Changed `IN` to `ON` in all note sentences in guides.

Closes https://github.com/rails/rails/issues/18372

[ci skip]

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

This config setting has been available for quite a while but was not
documented.

References #18148.