Merge branch 'master' into 'master'

make it really easy to upgrade unsigned repos to signed

As a key step to removing support for unsigned repos from fdroidclient (https://gitlab.com/fdroid/fdroidclient/issues/12), this merge request makes `fdroid update` require a signing key.  If there is no keystore, it'll prompt the user to create one using `fdroid update --create-key`.

This closes #13

See merge request !48

diff --cc fdroidserver/common.py
Simple merge

diff --cc fdroidserver/update.py
Simple merge