Allow event organizers to disable event enrolments

Problem
There is no longer a way to hide enroll option on events.

Solution

• implement for the new form structure
• should be possible for SM to decide if
  • enrolment can be disabled or not per event base
  • enrolment is disabled platform-wide
• If enrolment is enabled per event base, event creator should be able to enable / disable it
• If enrolment is disabled platform-wide, no enrolment options should be shown on edit form as well as event page