Formalize release process

This flatpak is often installed on Endless OS, which includes additional system integration in the host via [eos-kolibri](https://github.com/endlessm/eos-kolibri). As a result, we need to do an additional round of integration testing for any large changes to the flatpak. To achieve this, we have added a [`beta`](https://github.com/flathub/org.learningequality.Kolibri/tree/beta) branch, and test images of Endless OS use `org.learningequality.Kolibri//beta`.

For this to work, it is important that the beta branch is always _equal to_ or _ahead of_ the master branch. We want this testing workflow to be reliable. So, let's establish a release process which guarantees we generate a beta release before pushing to master; that the beta branch isn't missing changes from the master branch, and that major changes are always visible _first_ in EOS test builds.