Allow easy plug-in any storage backend

[d1skort]

For now, we are only using a MongoDB, but I believe there are no restrictions on supporting any other storage system.

Roadmap for doing this:

• [x] Research FerretDB for ad-hoc solution with postgresql
• [ ] Clean up dataAccess package. There are a lot of unused code and abstractions
• [ ] TODO

[danslapman]

I think we could implement #20 using this mechanism as well