Anti-Feature Maintainer-Notes as structured text

Currently anti-features look like this:

AntiFeatures:
  - Tracking
  - NonFreeNet

The basic idea is to additionally allow anti-features to have explanations texts eg:

AntiFeatures:
  - Tracking: |-
        sends crash-reports containing data userdata without asking for
        permission to do so
  - NonFreeNet: This app requires you to have an account registered with exmaple.com

The other half here is passing these texts on fdroidclient via index-v1. That's how anti-features currently look in there:

"antiFeatures": [
    "NonFreeAdd",
    "NonFreeNet"
],

Valid for both apps[].antiFeatures and packages[].appId[].antiFeatures. It's noteworthy that packages-entries don't contain any localized values right now. We probably should also allow antiFeatures in localized to make them translate-able.

(following up on !662 (closed))