Improving the way we build nightly apps

[Matthias Clasen <matthias clasen gmail com>]

So far, we have this gnome-nightly-apps repository which contains copies of the flatpak manifests for a bunch of gnome apps. That is redundant and suboptimal. Recently, flatpak has gained the ability to find manifests in git repositories that you point it to, and we should use this to move the json files to each applications git tree. Quite possibly there is already a copy of it there anyway.

If you want to help out with making this happen, the details are described here:

https://wiki.gnome.org/Initiatives/GnomeGoals/FlatpakManifests