Automate release management

[jo-elimu]

Facilitate automated releases triggered via Jenkins.

• [ ] Tag in GitHub
• [ ] Release (and upload signed APK) in GitHub
• [ ] Upload AAR (and APK?) file to Bintray - https://github.com/bintray/gradle-bintray-plugin#readme
• [ ] Upload APK file to website (add REST API endpoint)
• [ ] Automatically adjust version in build.gradle

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Relates to https://github.com/elimu-ai/ml-authentication/issues/147

[jo-elimu]

See https://github.com/chrisbanes/gradle-mvn-push