oauth: switch to custom uri scheme over localhost redirect

Google is deprecating/removing support for localhost redirects when authorizing an oauth client. So we need to switch to a custom URI scheme like 'org.dejadup:' and registering that with the system in our desktop file.

This has pros and cons in terms of UX. Some browsers (like Firefox) may prompt "what app do you want to use for this org.dejadup link?" which is a little gross.

But on the plus side, we now get to skip the "auth complete, close this tab and go back to Backups" screen and just immediately raise our window when the auth is done.

Fixes: #222 (closed)