Using ActiveSupport::Concern instead of plain regular Ruby hooks in railties/lib/rails/generators/migration.rb file

Remove redundant escapes from xml serialization test

Further clean-up of ActiveSupport::Callbacks

In #11195 I noticed a trailing comma in the docs, but I decided to
further clean it up.

What I have done:

* Clean up the trailing comma in the docs and some extra whitespace
  lines.
* Used `Array#extract` options to factor the repetitive pattern of
  `args.last.is_a(Hash) ? ...`
* Renamed the local var `config` to `options` in `define_callbacks`, as
  `options` seems to be the de facto name for the options objects.
* Renamed the local var `l` to `line` in `define_callback` (maybe it
  meant `lambda` in the context) as single `l` may look like `1` in some
  fonts.

Created way of cloning a Rails::Application instance

This change provides the ability to create a new application with a
configuration which can be specified.

updated AS:Callbacks docs, variable namings

Using preferred find_by syntax in guides

Adding find_by in guide to methods that trigger after_find

_ Rename the define_callbacks params to `names`
  - in order to match the naming conventions for `get_callbacks` and `set_callbacks` at https://github.com/rails/rails/blob/master/activesupport/lib/active_support/callbacks.rb#L736-743
  - `define_callbacks` just register names(events), not define the real callback functions.
- Rename the `reset_callbacks` params

Store a symbol directly inside DISPATCH_CACHE for additional roflscale

Remove evals from AM::Validations::Callbacks

Synchronize PostController#create code from 5.6 to others sections in Getting Started guide.

follow the same refactor at a63a964a

Fixed `permit` using for comments.

Remove a duplicated section [ci skip]

Added clarification for destroy link_to description in 5.14

Typo in Active Record Migrations Guide

The change is commited at ba552764

Attachments were previously covered so remove the useless part. Just
move a note about multipart headers set when the mail method is
triggered to the kept section.

Fix `rails plugin --help`

Right now if you run the `rails plugin --help` command it fails because rails expects a command in `railties/lib/rails/commands/plugin.rb` that does not exist because the file is named `plugin_new`. This is the error:

```
ruby-2.0.0-p0  ~/documents/projects/tmp/vanilla (master)
$ rails plugin --help
/Users/schneems/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `require': cannot load such file -- rails/commands/plugin (LoadError)
	from /Users/schneems/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `block in require'
	from /Users/schneems/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:213:in `load_dependency'
	from /Users/schneems/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `require'
	from /Users/schneems/.rvm/gems/ruby-2.0.0-p0/gems/railties-4.0.0/lib/rails/commands.rb:49:in `<top (required)>'
	from bin/rails:4:in `require'
	from bin/rails:4:in `<main>'
```

provide a more sementic local variables name in ActiveModel::Validation::Clusivity

Fetch value(s) from stringified options in tags that stringify options

provide a more sementicthe local variables name for ActiveModel::Validations::Clusivity#include? method
the original name `exclusion` is a bit confusing when using with the method `inclusion_method`
rename it to a more logic neutral name.

remove deprecated implicit join references

Update a broken link into the fixtures' template [ci skip]