Prevent to reserve seats or signup is enrolment is not open

- Enrolment is closed if enrolment_start_time is defined and is in the future or enrolment_end_time is defined and is in the past
- Show different warning message if enrolment is open but there is no available seats
- Add reserved seats to available seats amount if there is already reservation in the session storage
- Disable enrolment form also if there is no available places and seats reservation is not stored to session storage

Revert "Prevent to reserve seats or signup is enrolment is not open"

This reverts commit adeed9b.