Only show own repositories on the index page for teachers

[jorg-vr]

This pull request limits the repository index page to own repositories for non zeus users.

It is only limited on the index page, and not in the policy, as teachers are allowed to see the list of all repositories on other pages. (Eg as filter dropdown to search for exercises)

I also added an empty state to the repository page as this is now relevant.

part of #4362

[bmesuere]

I slightly prefer having a _html empty message with a <br> instead of the current solution with a \n and changing the css behaviour.

Also, I think technically oefeningenrepository's should be one word. Because you're allowed to add a dash to improve readability, I would make it oefeningen-repository's. But I'll let @niknetniko decide.

[jorg-vr]

Ik heb de \n vervangen door _html en <br\>. Maar de white-space css aanpassen blijft nodig voor alle text proper te tonen op kleine schermen.