Default I18n.enforce_available_locales to true

We will default this option to true from now on to ensure users properly handle their list of available locales whenever necessary. This option was added as a security measure and thus Rails will follow it defaulting to secure option.

Also improve the handling of I18n config options in its railtie, taking the new enforce_available_locales option into account, by setting it as the last one in the process. This ensures no other configuration will trigger a deprecation warning due to that setting.

Conflicts:
	actionview/test/abstract_unit.rb
	activesupport/CHANGELOG.md
	activesupport/activesupport.gemspec
	activesupport/lib/active_support/i18n_railtie.rb
	activesupport/test/abstract_unit.rb
	guides/source/upgrading_ruby_on_rails.md
	railties/test/application/initializers/i18n_test.rb