Add Radar-App

Application for showing alternative events from https://radar.squat.net site.

• The app complies with the inclusion criteria
• The original app author has been notified (and does not oppose 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

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Closes rfp#1837 (closed)