support uploading APKs to virustotal

https://virustotal.com is Google's free (as in beer) virus scanning service. It caches scans based on APK hash, so it would be good to have all of a repo's APKs uploaded to virustotal so that when users scan their own installed APKs, virustotal will report it already knows about that APK, and will rapidly report the results (otherwise, users can upload APKs for scanning via the web page or the free as in beer Android app).

I propose to make two new config options: the API key info needed to upload to virustotal. Then when those are set in config.py, fdroid server update will automatically upload all new APKs to virustotal.

There is a public API for doing all that with virustotal: https://www.virustotal.com/en/documentation/public-api/