This PR fixes I18n.locale for rake middlewares. Mastodon uses Devise that depends on Warden.
Warden::Manager can be found in rake middleware. It is outside of the controller.

In the case of authentication failed, warden calls throw(:warden). At the time Warden::Manager
delegates request to failure_app to generate response and flash[:alert] after catching it.
Unfortunately, I18n.locale is already reset then because I18n.with_locale is enabled only
inside the controller. If we used I18n.locale=, Devise::FailureApp could get the current locale.

* Add locale spec with failing locale plus region check

* Use a more accurate locale when supplied by browser headers

Previously we were using a matching option which would use the first locale
available which matched the locale portion, even if a region was specified.

This changes to first try to find an exact match, and then fall back to the
region, and then fall back to the  default.

* Clean up default_locale method

* Remove trailing whitespace

* Use query methods instead of explicit .blank? checks

* Use HTTP Accept-Language to detect locale

* Fix gem order to comply with codeclimate

* Sort gem to comply with rubocop

* I18n.default_locale fallback when there is no accept-language header

Setting of locale in controller extracted to Localized concern,
the doorkeeper authorized applications controller moved under
custom namespace with inclusion of Localized, which resolves the
"it sometimes appears in a different random language" bug