first proposal on Auto-Delete exercises

[anonym-HPI]

PR Checklist

Please make sure to fulfil the following conditions before marking this PR ready for review:

• If this PR adds or changes features or fixes bugs, this has been added to the changelog
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[anonym-HPI]

@Dassderdie @ClFeSc What do you think of this way to implement this?

The discussion before is also here: #183

@Dassderdie You wrote about setTimeout, I did use it in some kind (used the existing PeriodicEventHandler), this way the server could be set to like interval over every exercise like once a day and delete exercises that have not been accessed in a while.

Also the question is how often the sinceExerciseWithoutClients should be updated (only when no client is connected anymore or every minute if at least one client is connected, of course it should be renamed then).

[anonym-HPI]

If somebody wants to help write a test.
A test should do the following

1. Create a new exercise
2. Let a client connect to it
3. Disconnect this client
4. Wait at least Config.cleanupTime (maybe a bit more, to be sure)
5. Then try to connect again to this exercise
6. Expect that an error is returned that the exercise does not exist anymore

Another test could be:

1. An exercise is importet (without any client connecting to the exercise)
2. Follow step 4-6 above

Another test could be to check that an exercise is not deleted when CleanupEnabled === false