In from_redis method, statuses retrieved from the database was mapped to
the IDs retrieved from Redis. It was equivalent to order from high to low
because those IDs are sorted in the same order.
Statuses are ordered with the ID by default, so we do not have to reorder.
Sorting statuses in the database is even faster since the IDs are indexed
with B-tree.

* Fix #2619 - When redis feed is empty, fall back to database

* Use redis value to return feed from database only while RegenerationWorker
hasn't finished running

* Fix specs

* Replace usage of reject!

to the API

Also fix reloading of timelines after merge-type events

as well as formatting of content

and atom feed generation to some extent, as well as the way mentions are
stored