Add a way to sort out mirrors from the "explore" page

[564dtretrytr]

This is a bit of a problem when there are a lot of mirrors.

[IzzySoft]

Especially they should not be listed on the "Repository" tab when there's a specific "Mirror" tab for them. It's a bit confusing. Of course, I'm fine if it's at least configurable 😸

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs during the next 2 weeks. Thank you for your contributions.

[IzzySoft]

For the "Explore" page, this issue is still unsolved: it only lets you browse "repositories" – regardless whether mirrors, forks or originals. There's no way to show only forks, only mirrors, or only originals.