Allow logged in users to add assets as favorites

[Calinou]

Add an "Add as favorite" button and a way to list favorite assets for logged in users.

This would only be exposed in the web interface, not in the Godot editor since the editor can't log into the asset library yet. (Implementing login support in the Godot editor is possible, but far from trivial.)

[dark-penguin]

If we get global addons done right, then we won't need favorites in the Godot editor! Because you can simply download them all.