make raffle availability event specific

[jandd]

Is your feature request related to a problem? Please describe.
Currently the website shows the "take part in the raffle" checkbox for all events that are not ended. The raffle availability should be a configurable flag for an event instead.

Describe the solution you'd like

• Add a new boolean field "raffle_available" to the Event model
• add database migration to add the field with default value of "false" to existing events
• modify event.models.Event.is_raffle_available to check for the value of this flag
• add/modify tests

Describe alternatives you've considered
Just ignore the checkbox value on attendee entries if there is no raffle planned