More direct APK download links from GitHub

[freaktechnik]

This is a suggestion that we could use more user-friendly download links for the APK by just using slightly different URLs on GitHub:

• https://github.com/thunderbird/thunderbird-android/releases/latest to go to the latest full release, but not sure if this could also be a K-9 release?
• https://github.com/thunderbird/thunderbird-android/releases?q=prerelease%3Afalse to filter out pre-releases (and then potentially with it true as explicit link to the beta?)

cc @kewisch

[MelissaAutumn]

That sounds good to me, I'll hold off until we know if /latest includes k9.

[kewisch]

Right now, latest could be a k-9 mail release. I'd like that to change, it is essentially race conditions in our release automation. I don't think there is a workaround now. I can certainly change this manually after we release, but I might forget.