Flatpak improvements

[matthijskooijman]

While preparing for the 3.0.3 release, I encounted some things in the flatpak build that could be improved:

• Update flatpak file to better match https://github.com/flathub/org.gnome.Hamster/commits/master/org.gnome.Hamster.yaml (where appropriate - we cannot just copy the file as-is)
• Move flatpak file into data (and put .flatpak in the filename)?
• Maybe add provides to metainfo for dbus and python module? https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-provides
• Add content_rating tag to metainfo (see Update org.gnome.Hamster.GUI.metainfo.xml #690)
• Document use of --install-deps-from=flathub in README to autoinstall gnome runtime and sdk. This also prevents these dependencies from being pinned (which happens when you manually install them), so they can be automatically removed when unused.
• Also use --install-deps-from=flathub in the github workflow, so that no longer needs to hardcode the gnome version to be used (also listed in Github workflow improvements #744).
• Fix inconsistency in app ids (App ids don't match #725)