flatpak: Pull related refs

This MR fixes runtime related refs support in the Flatpak plugin in Ubuntu 18.04 GS branch. Without this fix, runtime related refs are not automatically installed/updated which results in Flatpak apps using wrong theme, wrong locale etc. and makes the user experience very bad. The support was fixed in GS 3.30 by switching to the new FlatpakTransaction API, which is unfortunately not easy to backport and requires recent version of Flatpak. This fix, on the other hand, works without any issue even with older Flatpak versions and is already used in Endless OS for some time. It was however not accepted upstream, because GNOME devs "have limited resources and have to focus on the new version of GS instead of fixing the older branch" and so it needs to be applied downstream. See the original MR and discussion in the Ubuntu bug report.

P.S. As recommended by @robert.ancell (see his comment), I did not include the test changes because they are not needed and make the MR much bigger.