[PATCH] Fix AppStream metadata

This makes the jami-gnome.appdata.xml pass appstream-util validate-relax and hence Flathub. The version number (the last bit required for Flathub) will be patched in from my side based on the version of the released source snapshot.

It also adds screenshots using the images from the new blog post and the website's features section. Can I rely on those URLs being stable for a while?