Use appstream-glib implementation of content rating systems
Move the content rating system implementation into appstream-glib so it can be shared by appstream-glib, gnome-software and libmalcontent-ui.
See https://github.com/hughsie/appstream-glib/pull/364.
This doesn’t add a dependency on the actual version of appstream-glib that we need (0.7.18) as it’s very modern. flatpak, for example, only depends on appstream-glib 0.5.x. We probably can’t bump our dependency until flatpak does, otherwise flatpak won’t be buildable with libmalcontent.