Add config to halt callback chain on return false

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

1 files changed, 2 insertions, 0 deletions

diff --git a/guides/source/configuring.md b/guides/source/configuring.md
index 0a375d7cb8..5baab8a4b5 100644
--- a/guides/source/configuring.md
+++ b/guides/source/configuring.md
@@ -507,6 +507,8 @@ There are a few configuration options available in Active Support:
 
 * `config.active_support.time_precision` sets the precision of JSON encoded time values. Defaults to `3`.
 
+* `config.active_support.halt_callback_chains_on_return_false` specifies whether ActiveRecord, ActiveModel and ActiveModel::Validations callback chains can be halted by returning `false` in a 'before' callback. Defaults to `true`.
+
 * `ActiveSupport::Logger.silencer` is set to `false` to disable the ability to silence logging in a block. The default is `true`.
 
 * `ActiveSupport::Cache::Store.logger` specifies the logger to use within cache store operations.