Optimise database access

[aumetra]

Currently every call to a mapping function will do a few calls to the database.
This would be fine if it's done for only one or two entities, but this is done for every post on a user's timeline, the outbox, etc.

Since we already have a Redis instance running, I'm thinking about abstracting away some of the database accesses behind something like the repository pattern and then doing the caching via the repository.