The images for group avatars are stored directly in the datbase, which
is fine since they are small. But then there's no reason to have them
lingering on the filesystem.
Also removed the unused group associatedAttachmentId property.
// FREEBIE
What we *previously* refered to as attachments are actually just the
attachment's id (NSString). This has tripped me up a few too many
times.
Also, use generics with attachment id's array.
// FREEBIE
