Non-existent users as signatory

[adehnert]

See ​https://diswww.mit.edu/menelaus/asa-db/820:

Traceback (most recent call last):

  File "/usr/lib/python2.7/site-packages/django/core/handlers/base.py", line 111, in get_response
    response = callback(request, *callback_args, **callback_kwargs)

  File "/mit/asa-db/Scripts/django/asa-db/asadb/groups/views.py", line 654, in manage_officers
    if role.require_student and not moira_accounts[person].is_student():

KeyError: u'abdimom'

[adehnert]

commit ce5d021f8088e9aa3c2ba6e6c61995903234b8fd
Author: Alex Dehnert <adehnert@mit.edu>
Date:   Sun Feb 26 02:28:25 2012 -0500

    Handle non-existent users on people page (Trac: #69)
    
    Fixes a 500 when people try to modify signatories where one or more
    of the involved people in the group no longer has a valid Athena account.