Migrate to standalone database

[muety]

Using bbolt with bolthold for minimalism was a cool experiment. However, we should switch to an actual database. One main reason is the ability to manually view and edit data from external applications or command-line, e.g. for debugging purposes. Another reason is performance, although that is not yet critical.

I'd either go for MongoDB, because it has already proven to work extremely well in other projects and is modeled after documents. A relational database would be a safe bet, but also require some refactoring in the data schema.