Open AppCenter listing if remote is already present

If you're opening a Flatpakref for an app that's in a remote you already have on the system, we could just pass you off to an `appstream://` URL so you never see a Sideload dialog. Then we handle installation inside of AppCenter just like if the user had browsed there themself.

This enables us to have a halfway-decent experience if browsing a Flathub-like web-based AppCenter site with Flatpakrefs (before we do a proper URL handler), while serving the same flatpakrefs to other Linux-based OS users.