per-user inbox & favorites

[acid angel from asia]

• create user inbox & favorites collections;playlists on user creation
• add a PERSONAL type for playlists & collections
  • add a user_id column with a check that it is not null if type is PERSONAL
• store their IDs on the user table
• alter frontend to use the current user's inbox & favorites instead of hardcoded IDs 1 & 2

follow-up:

• add an option to the collection/playlist choosers to include/exclude type = PERSONAL && user_id != my_user_id collections