Optionally filter out anti-features

Either enabled or disabled by default, we should offer the option to hide apps with certain anti-features.

This used to be the case in very old versions of F-Droid prior to content providers. I think they got dropped because they were non-trivial to port over to the new system, and making F-Droid not crash due to out-of-memory exceptions was more important.

@pserwylo since you wrote the whole the whole content provider stuff, perhaps you have an idea on how to implement this? If you explain it briefly, maybe someone else will provide a patch.