BUG: Booking places in past competitions

[dompatmore]

Given:

A secretary wishes to book a number of places for a competition

When:

They book a number of places on a competition that has happened in the past

Then:

They receive a confirmation message

Expected:

 They should not be able to book a place on a post-dated competition (but past competitions should be visible). 

The booking.html page should be displayed for a valid competition.

An error message is displayed when a competition is invalid and a confirmation message is displayed when a competition is valid.