Reload exam before opening from esf in student view

The table in the esf might hold stale data when the exam was changed.
The is mostly relevant for debugging/testing purposes.
Respect subscription and unsubscription period on student view

It seems this check got lost durin some rework in the past. Now added back:
The (un)subscription period is checked immediately before the user is
(un)subscribed. Additionally the corresponding UI elements are only shown when they
will not result in an error message.