Add moderator notes for users

[aapeliv]

The admins/moderators need to keep track of why people have been banned, or sometimes other private notes.

Proposed idea:

• Add a text field to models.User, something like admin_note
• Add admin_note field to org.couchers.admin.Admin.BanUser
• Add an API call org.couchers.admin.Admin.AddAdminNote
• Return the admin note in org.couchers.admin.Admin.UserDetails

On adding a note, it should include a timestamp and the admin's id+username, and append to the note. Alternatively add a new table of AdminNotes or something and store it there instead of hacking around with concatenated strings. (Though I think concatenating strings is quite enough.)