Unrequire fields for Exec

[adehnert]

Almost all fields should be optional when an Exec member is doing the editing. We already futz with what fields are required (groups/views.py, GroupChangeMainForm?, line 122ish). We should futz more to mark fields as *not* required when an Exec member is doing the editing.

[adehnert]

The admin has long required substantially fewer fields than the normal UI, and is conveniently linked from the normal group page. For the non-admin interface, I like to keep parity for Exec and groups, in general, because it makes it easier for Exec members to provide correct instructions on how to use the DB.

I unrequired two more fields in the admin in:

commit 54babcd3060a9a66726ea88cf50f5efc6d4fcfbc
Author: Alex Dehnert <adehnert@mit.edu>
Date:   Mon Sep 17 02:53:03 2012 -0400

    Unrequire description, website_url in admin
    
    The only three fields that group members can edit (and thus that it makes
    sense for Exec members to punt updating so that group members put correct
    values in) are description, website_url, and officer_email. officer_email is
    filled in for all but two (derecognized) groups, so it seems like it makes
    sense to prevent Exec from accidentally unsetting it. description and
    website_url are frequently not set, so unrequire them.