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, 22 insertions, 0 deletions

diff --git a/guides/source/upgrading_ruby_on_rails.md b/guides/source/upgrading_ruby_on_rails.md
index 0b9f59bb46..c0c94475e0 100644
--- a/guides/source/upgrading_ruby_on_rails.md
+++ b/guides/source/upgrading_ruby_on_rails.md
@@ -53,6 +53,28 @@ Don't forget to review the difference, to see if there were any unexpected chang
 Upgrading from Rails 4.2 to Rails 5.0
 -------------------------------------
 
+### Halting callback chains by returning `false`
+
+In Rails 4.2, when a 'before' callback returns `false` in ActiveRecord,
+ActiveModel and ActiveModel::Validations, then the entire callback chain
+is halted. In other words, successive 'before' callbacks are not executed,
+and neither is the action wrapped in callbacks.
+
+In Rails 5.0, returning `false` in a callback will not have this side effect
+of halting the callback chain. Instead, callback chains must be explicitly
+halted by calling `throw(:abort)`.
+
+When you upgrade from Rails 4.2 to Rails 5.0, returning `false` in a callback
+will still halt the callback chain, but you will receive a deprecation warning
+about this upcoming change.
+
+When you are ready, you can opt into the new behavior and remove the deprecation
+warning by adding the following configuration to your `config/application.rb`:
+
+    config.active_support.halt_callback_chains_on_return_false = false
+
+See [#17227](https://github.com/rails/rails/pull/17227) for more details.
+
 Upgrading from Rails 4.1 to Rails 4.2
 -------------------------------------