Allow a go server to initiate account linking

[anoek]

While considering #88, please also consider adding of accounts from "the other direction", not only through the usgo online-ratings interface.

As a user, I should be able to go to some settings page on my favorite go server and link my AGA account to my go server account. This should be as streamlined and as simple as possible. For example, entering their AGA ID and clicking "Link". This action would then presumably forward them to a authentication or authorization page in the online-ratings system that would hold the users hand while finishing the account linking process. The server should be able to send along all of the necessary server-specific information, so all the user should need to do is click that button, login or create an account, maybe click a confirmation of "Yes link this account", and be done.

Letting the go server know the linking happened, via a post back, or simply relying on a user redirect back to the server, would be a great bonus.