Setup Flathub beta channel for .9x releases
Per Alex's blog post: https://blogs.gnome.org/alexl/2019/02/19/changes-in-flathub-land/ Flathub now supports a beta channel for publishing pre-release builds for wider testing.
After 0.13's bumpy release, and now that the plan is to start sicking to GNOME's release schedule (which calls for a number of uneven-numbered unstable releases), it would be good to publish the unstable releases to Flathub's beta channel to get new releases tested more widely, and to give people a means to fall back to a stable version if needed.