Add recovery code support for two-factor auth (#1773)

* 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

config/locales/simple_form.en.yml

@@ -10,6 +10,8 @@ en:
         note: At most 160 characters
       imports:
         data: CSV file exported from another Mastodon instance
+      sessions:
+        otp: Enter the Two-factor code from your phone or use one of your recovery codes.
     labels:
       defaults:
         avatar: Avatar