Create and use a flatpak manifest for a Kolibri base app

[dbnicholson]

From the org.learningequality.Kolibri flatpak manifest, split Kolibri itself, Kolibri-specific tools, and tasks related to cleaning up Kolibri's files, into a separate flatpak manifest: org.learningequality.Kolibri.BaseApp.

• I believe we can remove kolibri-home-template.json at this stage, as Kolibri's included pre-provisioned home should be fine. This will need to be tested.
• Submit the new base app to Flathub.
• After we update the original flatpak manifest, it should depend on the base app. Hopefully it will only need to add kolibri-gnome (built with default options), extension points, and a few dependencies.