Allow users to add or remove themselves to/from groups using the API.
Previously, there was no way for a user to add themselves to a group, or

remove themselves from a group. Only admins were able to do this. There

was also a bad query preventing adding on non-localsites.
Now a user can add themselves to any group they'd have access to, and

remove themselves from any group they're on. This works with both global

groups and groups on Local Sites.

Unit tests pass.
Added myself to a global group as a non-admin, and removed myself from

the same group.