Explicit status for paid and pending-payment

I normally prefer to calculate things that can be calculated, but with other statuses explicitly stored, managing this was becoming increasingly burdensone. So was having different value formats in database and in RegistrationForm.

This commit unifies all status handling to explicit values stored in the database.
New CANCELLED status for Registrations
Color adjustments for CANCELLED status

PENDING-REVIEW now becomes blue, as black will be reserved for the effectively dead, cancelled registrations.
Displaying Cancelled counts in statistics
Cancelled info strings for the Review
Cancelled registrations cannot get discount codes
Message for cancelled registrations trying to pay
Excluding non-paid or pending statuses from stats
Merge branch 'status-cancelled-paid'
Disallow NULLs for status collumn

We have explicit values for all statues now, so NULL is neither meaningful
nor needed and would in fact be an error.