config option to enable per-app repos for nightly builds

For Guardian Project, we've been running an fdroid repo for the nightly builds for each of our apps: https://dev.guardianproject.info/debug This is built using a big, hacked up shell script: update-debug-fdroid-repo. It has proven very useful to us to be able to subscribe to the nightly build for a single app, so this the first step of porting that horrid shell script to fdroidserver.

This also helps make the fdroidserver tool suite the single set of tools for all types of builds and releases. That will hopefully drive more free software developers to make f-droid.org the core channel for official releases.