Provide downoadable apk releases on the website

[MrCsabaToth]

Originally filed for the app by @ippocratis TrackMyIndoorWorkout/TrackMyIndoorWorkout#519
This could become part of the release procedure to make the latest build available. Probably we should provide one apk for the recent beta and the recent production version as well (unless the two match).

[MrCsabaToth]

We'll start to add binary releases in the app's repository. The question is where and how we will reference those from the website.

[MrCsabaToth]

This will make the whole app more professional! https://github.com/TrackMyIndoorWorkout/TrackMyIndoorWorkout/releases