This is kind of a dirty fix. I suspect that the MesssageForm class gets
used elsewhere (like for contact forms and the like), so if that's the
case, we may need to make a specific PresentationProposalForm class and
use that in `conference_main/views/event.py` (where the form is actually
built).
It's now possible to add Sponsors to an Edition. Sponsors are added
to the template context as 'edition_sponsors' as a dictionary sorted
by SponsorLevel order.
