Initial version of OAuth authentication

Motivated with a chat with user on IRC, who mentioned the plain text passwords in Windows registry, which is something that I should have fixed a long time ago.

This is not yet finished from UI perspective, but it's fully functional.

I'd like to get some feedback on would people expect the authorization process work. I'm not really good at UI work, so I made a very crude version first. It just opens the browser and a minimal popup that expects that authorization code. There isn't any progress or error reporting. 

We could probably also use the HTTP server we have on localhost, but I'm not sure if that's actually better than copy&paste of the authorization code and it's way more work as we have to make the page nice.

Also, if you are not logged in, Picard asks you for username/password but never saves those. Should we disallow the prompt for MB?