Add apks to github release

[ztefanie]

Is your feature request related to a problem? Please describe.
It should be easier for users to get our latest apk without using google play store.

Describe the solution you'd like
The apk should be added to the github release. Use octokit for adding it.

Additional context
We want a link that always links to the latest release for every app, e.g. github.com/digitalfabrik/integreat-app/latest/integreat.apk. See here: https://docs.github.com/en/repositories/releasing-projects-on-github/linking-to-releases
This link can be placed on e.g. the website.

Think about how to handle betas, they should not be latest, but get a "pre-release" tag or something, check if the linking would be still correct.