F-Droid Basic 2.0

This is a massive MR that aims to merge the entire 2.0 development effort into the official repo. All new code was added to a new gradle module to not mix both codebases. The goal is to eventually remove the gradle module containing the old code when the last bits have been rewritten or migrated to the new codebase.

Note that getting this merged isn't a release. Before doing the first alpha, we can still make changes.

Wiki-page over the new design changes: UI overhaul