Enforce max locker name length

[adehnert]

The new group form should enforce the twelve character maximum length for lockers (​https://diswww.mit.edu/menelaus/asa-db/11927).

[adehnert]

Alleviated in commit 8ccd88fca9e25f1d1bbe056a7468586546235a8f:

Author: Alex Dehnert <adehnert@mit.edu>
Date:   Mon Jun 3 05:33:53 2013 -0400

    Add help text about Athena lockername selection
    
    As mentioned in ASA-#234, some people starting groups pick excessively long
    locker names. This adds help text to try to alleviate that tendency (and also
    to reduce the number of invalid characters people submit).
    
    Unfortunately, enforcing this well seems difficult --- we want the SQL database
    to support longer lockernames (some groups have org lockers instead of activity
    lockers, and can therefore have longer names, for one), but have the frontend
    (sometimes) reject it. It's not clear how to do this in Django.

[adehnert]

Fixed(ish) in commit 7dde669e81610203605da103ecc9e9abb5fa0bd4 by adding a warning framework. Hopefully people will pay attention.

Author: Alex Dehnert <adehnert@mit.edu>
Date:   Sat Nov 30 16:36:14 2013 -0500

    Add pre-group-approval warnings
    
    This fixes (enough, at least for now) several issues by adding warnings
    to the approval page when certain constraints are violated:
    - ASA-#216: New group "ERROR:" emails should get suppressed earlier
    - ASA-#234: max locker name length
    - ASA-#246: Add reminders of common issues to the new group confirmation page