Implement chesscom support

[teotwaki]

It would be great to be able to support chesscom in addition to lichess.

From an API perspective, chesscom is a bit more limited, however it's not unmanageable:

• There's no OAuth support, however one common technique is that as a challenge/response system, we give the user a token they should put in the Location field of their profile. Once they validate, we fetch the profile and verify the token is there. It requires a bit more manual work on the user's part, however it does prove account ownership.
• chesscom has an API that we can access to fetch ratings and such. It's by far not the sexiest of APIs to work with, but it will get the job done.

Questions:

• how do we handle people with both a lichess and a chesscom account? Do we average the ratings? Do we show the highest? Should we handle both accounts at all?
• How do we show the difference in ratings? Do we need to show the difference at all? We could have one role that says "Liro Lichess" or "Liro ChessCom", so that we don't need to duplicate all the roles again (this only works with the single-account-linked approach).

[teotwaki]

Note: Chesscom has implemented OAuth, however they require credentials in order to invoke it. I've requested said credentials, and have a branch ready to go once I can test things.

[teotwaki]

Update: still waiting for chesscom to provide me with credentials.

This is why we can't have nice things.