per-user database

The intention of this branch is to move all user-related data into one database per user.

Benefits
* possibility to delete a user without leaving any traces
* possibility to migrate individual users between servers
* sharding

TODO
- [x] split design documents into user and general
- [ ] reads / write of user-specific data should use user-db
- [ ] adapt commands that aggregate data: update-toplist, update-episode-toplist
- [ ] move chapters from UserEpisodeState into Episode (additionally avoids one db query per episode page view)
- [ ] aggregate podcast lists (or keep them in central database)