Add a "Flagged" category

There is currently no clear way to differentiate apps of questionable status, which:
- Have no active development
- Fail to install at all, faulty & dangerous
- Have only a single release on the appstore
- Are X versions behind current Nextcloud stable.

- Can become part of a way for people registered on the appstore to flag or report (or simply identify) these apps:
- For complete removal from the appstore
- Author has requested removal on their git repo or elsewhere
- Active fork has appeared, which is also on the appstore, and the previous app is clearly abandoned

## Flagged category if nothing else is a stop gap solution
- Any app added to "Flagged" category could at least be easily identified by being in Flagged.
- You could remove a flagged app from all other categories, so it is only visible in Flagged or All.
- Flagged apps could also be worth making a different color or appending / prepending text to make it totally clear the app is flagged at a glance within the appstore.
- Add an auto-pinned review on any Flagged app to specify: "This app has been flagged as possibly abandoned or unstable."
- Flagged apps could be treated as having no rating, to further clarify they are flagged and questionable
- See #1472

Hope this inspires some ideas. Cheers! Cleaning up the appstore listings could clear out the entropy.
See also
- #412
- #566