Leftovers in order to ship Firefox as flatpak for the https://flathub.org/ appstore.
Build fixes in order to get a MVP
- to rope in legal + others teams @Mozilla when we generate https://hughsie.github.io/oars/generate.html
- description needs to change - it's terrible ATM - check what we have on Google Play store
- check formatting options for HTML rendering here - https://www.freedesktop.org/software/appstream/docs/
- use https://github.com/flathub/flathub/wiki/AppData-Guidelines
- check license tag (is it MPL-2.0 indeed?) + rope legal into this
- review the fields of the appdata.xml alttogether (summary and such, etc)