* Add confirmation step for email changes

This adds a confirmation step for email changes of existing users.
Like the initial account confirmation, a confirmation link is sent
to the new address.

Additionally, a notification is sent to the existing address when
the change is initiated. This message includes instruction to reset
the password immediately or to contact the instance admin if the
change was not initiated by the account owner.

Fixes #3871

* Add review fixes

Force SSL only cookies for remember_me, adjust confirmation
expiration time to fit with the user cleanup scheduler

* Fix #4058 - Use a long-lived cookie to keep track of user-level sessions

* Fix tests, smooth migrate from previous session-based identifier

* Add overview of active sessions

* Better display of browser/platform name

* Improve how browser information is stored and displayed for sessions overview

* Fix test

* feat: Revocable sessions

* fix: Tests using sign_in

* feat: Configuration entry for the maximum number of session activations

* 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

Do not require e-mail address changes to be re-confirmed, it's only trouble for no real benefit

POST /api/statuses                  Params: status (text contents), in_reply_to_id (optional)
GET  /api/statuses/:id
POST /api/statuses/:id/reblog

GET  /api/accounts/:id
GET  /api/accounts/:id/following
GET  /api/accounts/:id/followers
POST /api/accounts/:id/follow
POST /api/accounts/:id/unfollow

POST /api/follows                  Params: uri (e.g. user@domain)

OAuth authentication is currently disabled, but the API can be used with HTTP Auth.