Fix multiple repository related paper cuts

[jorg-vr]

This pull request fixes multiple paper cuts related to repositories.

• The placeholder for the clone url on the repositories/new page mentions github.ugent.be. While correct, github.com would be more logical. From Paper cuts #4240

• When adding a repo, add a better error message when permissions are missing. From Paper cuts #4240
  New message:
  

• The learning activities table (at least the one on the repo page) has no empty state. From Paper cuts #4240
  New empty state:
  

• When adding a repository, the existing exercises were not added automatically. From Paper cuts #4240
  This issue was only present in staging and production, where the git worker queue allowed the tasks to be done simultaneously/out of order. This should now be forced to consecutive execution.

Part of #4362 .

[bmesuere]

I don't like the email solution either. Can't we just send an email to the address associated with the repo admin in this case? Since the repo has just been added by that user, using that address seems like a good fallback.

[jorg-vr]

the repo admin is only added after the repo is saved.
Because cloning the repo is a delayed task, and it also takes some time to perform, this admin will almost always be added before the start of process activities.
However, this order of execution is not at all guaranteed and could result into bugs in some cases. (Most notably it causes this bug in our test environment where deleyad_jobs are executed in order)

This was my first solution which I reverted because of a failing test. Do you want to continue with this approach? (In which case I would just remove the test because we know it will fail)

[bmesuere]

In that case, maybe add dodona@ugent.be as ultimate fallback?