Run some things as background tasks

[aumetra]

Right now we, for example, fetch previous posts in a thread synchronously when loading the "genesis" posts (i.e. the post the user requested).
Due to latency, slow networks, slow servers, and a bunch of other factors, we shouldn't run this synchronously but rather as a background task that can be picked up by a job runner.

Another candidate for this are link previews from #247. These previews are also fetched synchronously instead of asynchronously backfilling them in the background after a post has been created.