Archiving System for finished courses

[meepoSenpai]

Add some sort of archiving system, if the courses are already finished.

[JustusAdam]

Status added as of 3dd2a53.

Checking and filtering in all places where only active courses are used remains to be done.

[ManuelvOK]

Semi fixed it in b638c24 by filtering in all places where courses are listed. The following should still be done:

• buttons for manual archiving
• routines for automated archiving in relation to end date
• archived courses should be visible for admins, course teachers, and attendees
• archived courses should not be edited in any way by course teachers