Adds GroupInvite model, and group-related fields on some other models.

Also adds a proper merge migration for 0020_* in sleep, my bad on not
doing that before.
Adds SleeperGroup.invite() and GroupInvite.accept()/.reject().
Changes add to invite.
Adds accept/reject invite functionality and interfaces.
Adds number of group invites in the top bar.
Adds autoAcceptGroups to SleeperProfileForm.
Adds GroupInvite to admin.
Add autoAcceptGroups to editprofile template.
Adds area for dependencies, so we don't forget :D
Merge branch 'master' of https://github.com/sleepers-anonymous/zscore
Adds directories for ./manage.py custom commands
Merge branch 'master' into pretties