Handle mirrors in "Add repo" preview better
This MR includes changes to better handle the (user) mirrors in the "Add repo" preview flow.
Changes:
- Show the URL of the mirror being added (below the repo URL)
- If the mirror was already added, say "mirror" instead of "repository" in the red error message
- Add a small blurb text to explain that this is a mirror of another repo.
- Explain (in the button and in the new info text) that the upstream repo will be added, too, when you add a mirror without already having the upstream repo. To test this case, delete the F-Droid main repo, and then add it again by adding a user mirror.
See the individual commits, which should be fairly self-explanatory.
Screenshots
The first three screenshots show the changes. The last screenshot (adding the Guardian Project repo) has no change and is just for comparison).