DN should be used as the natural key for list fields

[zebulasampedro]

Right now the list fields for associating users with projects use only the username as a natural key. Given that usernames can be repeated across orgs users should instead be represented in these fields by their DN.

[zebulasampedro]

This will break expected functionality for project event mail notifiers, as the managers and collaborators fields will now render a list of DNs.

Models that store user lists will need a new set of properties to render correctly in the event notifiers.

[zebulasampedro]

Also the ListField will need to be amended to support non-comma delimiters, otherwise lists of DNs will be incorrectly parsed.

[zebulasampedro]

Many of the view functions map the request.user.username to managers and collaborators, which breaks now that managers and collaborators are identified by DN and not by username.