[FEATURE] add option for seats

[jpmschuler]

Powermail already offers a functionality to mark specific fields as "name" or "email" and allows to resolve them.

The same approach could be used to mark an (int) field as "seats" and thus allow a real calculation.

This will be delayed until the current set of functionality is deemed stable.

This will have two additional drawbacks to keep in mind:

• 8/10 seats taken, with waitlist active; - new registration for 3 seats => that registration is waitlist and list will be marked full.
• 8/10 seats taken, without waitlist - new registration for 3 seats will be declined => slots still free