* Add overview of active sessions

* Better display of browser/platform name

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

* Fix test

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

An attempt to open a brand new Mastodon instance configured
as SINGLE_USER_MODE=true will cause an exception.

Enable temporary registration if we have no users in the database

Fixes #1817

to sign in page after sign up instead of root path which redirects to /about

behaviour with Turbolinks enabled, optimizing Rabl for production