Merge branch 'per-app-repos' into 'master'

Merge branch 'per-app-repos' into 'master'

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](https://github.com/guardianproject/fdroid-repo-tools/blob/master/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.

See merge request !66