Add "flair" images.
Simplify header design for sites without flair image.
Enable /user-flair.jpg for logged-out users. Sorry!
Add (basic) server administration.

* Users with the (new) `admin` flag can create other users.
* `/server` now has a (very simple) dashboard with information about the local server instance.
* The footer now has a link to the server admin dashboard if the user is an admin.
* Existing users or posts can't be edited yet, but maybe in the future.
* General cleanup.
Fix TimelineManager to check if the given post and its user are present before doing its thing.
Fix sanity checks during user.json retrieval.
General service class cleanup.

* Move user fetching into new UserFetcher service
* Decouple fetching of user from fetching of posts
* Add AppSignal monitoring for background jobs

A couple of notes:

* The AppSignal integration is a bit convoluted right now. I'll figure out a nicer DSL eventually. I hope.
* UserFetcher needs specs. :/
Fix UserPoller.

It was polling way too often.