New scheme for room (replace ArbitraryStorage table and datastore)

[mahibi]

Currently the app uses datastore and room, while room only has 2 tables (User and ArbitraryStorage).
It's currently not clear when to use datastore(via AppPreferencesImpl) or ArbitraryStorage.

There should be only room used to store any data.
ArbitraryStorage should be replaced as it contains mixed usecases which is not good (user settings and conversation settings).

A new scheme must be created which for the beginning contains tables for:

• app wide settings
• user wide settings
• conversation settings

while always having in mind that in the long term we also want to have full offline support for conversations.