Add the `accept_proposals_at` on Event Form.

[luanfonceca]

We have added a field named accept_proposals_at on the Event model, according to the issue #304, its blocking by his progress, until someone implement it we can't do anything.

This issue aims just to add the new field on the event_form.html template, make it work with the Create and Update actions and change the necessary tests on test_functional.py.

[chawki27000]

I take this one