allow Anti-Features to include localized descriptions about why they were added

Right now, Anti-Features are just a tag. This adds the possibility of adding the tag, then a matching text to explain why the Anti-Feature was tagged. To get there, I refactored a lot of the custom TYPE_* handling and introduced a new type TYPE_STRINGMAP for Anti-Features.

closes #683 (closed)

I took this opportunity to do a lot of work towards making the internal data format closer to index-v2 #1107

Tests needed before merging:

• Full run of fdroid update on fdroid-buildserver-full-copy
• fdroid rewritemeta leaves fdroiddata unchanged.
• fdroid readmeta passes on https://gitlab.com/guardianproject/fdroid-metadata
• Before/After comparison of https://f-droid.org/repo/index-v2.json files.