Add -dev suffix to version and package name, use staging backend for debug builds

Changes

• Adds -DEV suffix to version name and .dev to application ID ("package name") for debug builds, allowing dev builds to be installed alongside the production app from the Play Store. The dev version of the app is listed as "tinyCampus DEV" in the app drawer.
• For debug builds, the staging backend is now used automatically.
• The production backend URL has been changed to https://api.tinycampus.de

Sources:

• https://stackoverflow.com/questions/18491649/how-to-change-the-android-app-package-name-when-assembling-with-gradle/21006552#21006552
• https://stackoverflow.com/questions/24785270/how-to-change-app-name-per-gradle-build-type/36455983#36455983

TODO

• iOS support (see #156)

See #102 (closed)