Make .apk downloadable from generic link

[hamaryns]

Please see https://www.bewelcome.org/group/60/forum/s23449: make it possible for people to be independent from Google by providing a download link to the latest apk. Make that a generic link that does not change, independent of the version number, for example, create a new folder "pwa/last", always put the latest version there and just call it BeWelcome.apk.
And automate this process, of course. And mention it in the faq and anywhere there is reference to the app.