Unique group names should not use random integer

[boonebgorges]

If there's a group foo and you try to create another group foo, your new group will have a slug like foo-43983498 - we append a random number to the end. See groups_check_slug(). This is terrible.

We should:

1. Increment (-2, -3, etc) instead of using a random number. This is much friendlier.
2. Add something to the UI that warns the user about the conflict and urges them to choose another name. (This should probably be a separate ticket.)