Improve gig enquiry -> gig interaction

The process for booking a gig is (initially) something like*:
- Client emails secretary
- Secretary replies saying yes with a quote, and creates a gig enquiry on the website with the quote details and the quote date
- Client pays the deposit and emails the sec to confirm this.
- Treasurer confirms that the deposit has been received. At this point, the booking should change from a gig enquiry to a gig (i.e. a confirmed booking).

My initial thoughts are there should be some validation to prevent enquiries being left as enquiries with the deposit marked as received. However, this may be slightly trickier since deposit received may be marked by the treasurer, but the secretary may want to be the one that marks the booking as confirmed, so just adding validation may not be the answer (and if we were to just add validation, we may as well make it so that ticking deposit received on a gig enquiry changes the gig type to gig and vice versa).

* Obviously we could say no, or the client decides not to go ahead with the gig and such, but that is irrelevant in this task.