Add support for event date ranges

The `Event` class has several date ranges associated with it. It has the
`is_registration_active` property to check whether or not registration
for the event is open. It's now getting the
`is_call_for_proposals_active` property to check whether or not the CFP
is active and accepting submissions.

Both of these properties are now being utilized to help populate the top
nav. `Event.is_call_for_proposals_active` is also being utilized to
prevent users from submitting new proposals to events outside of the
specified date range. Existing proposals can still be edited after the
close of the CFP.
Display Call for Proposals end date

When an end date has been specified for the CFP it should be displayed
on the CFP page.
Merge pull request #15 from PyGotham/respect-dates

Respect dates