Not accept proposals before the `accept_proposals_at` date.

This issues depends on #304.

if some event has defined the accept_proposals_at field, we can only accept proposals after that time, this should create some changes on the Models, Views and tests.

Tasks:

• Check if the accept_proposals_at date has passed.
• Restrict the cases that the proposal cant be created and return a Message for the user.
• Add tests to assert that the a proposal cannot be created after the date.