gitlab-ci: push nightly builds to fdroid repo on gitlab

This sets up a nightly build repo using fdroidserver's new gitlab repo hosting feature. Once this is merged, any build on fdroid/fdroidclient's master that passes all the tests will be published to https://gitlab.com/fdroid/fdroidclient-nightly

If a repo is set with the gitlab-ci Secret Variable DEBUG_KEYSTORE and there is a repo named the same as this repo with -nightly appended, then this will automatically generate an fdroid repo of each build produced by gitlab-ci runs on the master branch.

closes fdroidserver#256 (closed)