Basic GitHub account linking

[outoftime]

If the user is logged into an account that is not currently linked to a GitHub identity, display an option in the account drop-down to Link GitHub. Clicking that should prompt the user to link their GitHub account via OAuth pop-up.

Note that this does not contemplate merging in an existing account—that will come as a followup. For now, if the authorized GitHub identity is linked to a different existing Popcode account, just show an error message.