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
* split design documents into user and general: done
* 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 (own keep them in central database)