F-Droid requirements

[casimir]

F-Droid has a a list of requirements. Some are good for the project in the long term, some are just requirements.

• [x] fastlane in root directory → #173
• [x] Keep screenshots in git repo → #173
• [x] Maintain changelogs in fastlane metadata (difficult with the current release workflow) → manual
• [x] Use a pinned version of Flutter in CI → ~#175~ #177
• [x] Build isar from source using git submodule #178
• [x] Bundle Google Fonts → #172

Merge Request: https://gitlab.com/fdroid/fdroiddata/-/merge_requests/14933