* Fix regeneration marker not being removed after completion

* Return HTTP 206 from /api/v1/timelines/home if regeneration in progress
Prioritize RegenerationWorker by putting it into default queue

* Display loading indicator and poll home timeline while it regenerates

* Add graphic to regeneration message

* Make "not found" indicator consistent with home regeneration

* Add spec for precompute feed service

* Refactor PrecomputeFeedService

* spec wip

make regeneration worker unique, (only schedule/execute once at a time)

must be added to the Sidekiq invokation in your systemd file

The pull queue will handle link crawling, thread resolving, and OStatus
processing. Such tasks are more likely to hang for a longer time (due to
network requests) so it is more sensible to not make the "in-house" tasks
wait for them.