Loading icon for bridge pass (Lox) invites

In both the dialog for users to supply bridge pass invites, and for the dialog to generate their own invites, we have a loading state whilst we wait for the network response.

The mockup had a loading icon, which was not implemented in #42036 (closed).

@donuts the mockups had a purple loading icon. Should we just use the existing blue icon from Firefox? Icon was added in !988 (merged).