(You may think that we need account deletions, but this way would've just orphaned the db records)

* Add a spec for signing out

* Add spec showing that suspended user gets a 403 forbidden on sign out

* Allow suspended account users to sign out

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 recovery code support for two-factor auth

When users enable two-factor auth, the app now generates ten
single-use recovery codes. Users are encouraged to print the codes
and store them in a safe place.

The two-factor prompt during login now accepts both OTP codes and
recovery codes.

The two-factor settings UI allows users to regenerated lost
recovery codes. Users who have set up two-factor auth prior to
this feature being added can use it to generate recovery codes
for the first time.

Fixes #563 and fixes #987

* Set OTP_SECRET in test enviroment

* add missing .html to view file names

registration API