Update SecureToken Docs

Add Changelog entry for has_secure_token [ci skip]

Remove unneeded `require 'as/deprecation'`

Tests should still pass after removing `require 'active_support/deprecation'`
from these files since the related deprecations have been removed.

Add missing information regarding callbacks

Remove all deprecation code

These requires were added only to change deprecation message

Before this change we had a small set of "truthy", and all others
are "falsy".

Now, we have a small set of "falsy" values and all others are
"truthy" matching Ruby's semantics.

Before this change any error raised inside a transaction callback
are rescued and printed in the logs.

Now these errors are not rescue anymore and just bubble up,
as the other callbacks.

Now all strings will be handled as a URL.

This reverts commit ae96f229.

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/lib/active_record/attribute_methods.rb

Files without a template handler in their extension will be rended
using the raw handler instead of ERB.

doesn't contain `#`

These methods were deprecated in Rails 4.2 (see f4ee1147) so they can
be safely removed in Rails 5.0.