WIP: include the developer APK signature if available

Open username-removed-24982 requested to merge eighthave/fdroiddata:developer-signatures into master

This is part of the great scrape. It is the APK signature from the original app developer for all the relevant APKs that I could find. There are probably more available, but this is a good start. For reproducible builds, this signature will be used on the APK that f-droid.org ships.

for more info, see fdroidserver#291 (closed)