Add vertretungsplan.io App

• The app complies with the inclusion criteria
• The original app author has been notified (and supports the inclusion)
• All related fdroiddata and RFP issues have been referenced in this merge request
• The upstream app source code repo contains the app metadata (summary/description/images/changelog/etc) in a Fastlane or Triple-T folder structure
• Builds with fdroid build
• Releases are tagged

Metadata is provided using triple t; description is at https://codeberg.org/vertretungsplan/client-android/src/branch/master/app/src/main/play/listings/en-US/full-description.txt