Adds a confirmation modal that warns about unsaved changes
to the following tabs of the event details:
Metadata,
Extended Metadata,
Scheduling (for scheduled events),
Workflows (for scheduled events).
Thereby this fixes opencast#982.

Includes opencast#1046, because I was expecting this to be tightly related to
modals and did not want to write it twice. The general coding
approach would be the same though.

Speaking of approach, I would like feedback on if this makes
for a good approach, or if there are better alternatives I've
missed. If people agree with this approach it could then be
extended to other modals (e.g. series) as well.