Press "reject" on a group invitation; entered into the group anyway!

[Aaron Wells]

To replicate:

1. Create a group
2. Invite another user to join the group
3. Log in as the other user
4. Navigate to the group, or look at the group invitation notification in your inbox.
5. Click the "reject" button on the invitation

Expected result: You see "Invite declined successfully", and you do not join the group
Actual result: You see "Invite accepted successfully. You are now a group member." And you join the group.

Verified that this is present in 15.10 and 16.04, but not in 15.04.

[Aaron Wells (u-aaronw)]

"High priority" because it's so obviously broken, and potentially infuriating to the user. There is, however, a trivial workaround, which is that you can click "Leave this group" and immediately exit. So this bug mostly just looks bad.