Provide a way for staff to certify w3c/github link

[dontcallmedom]

In some cases, it proves difficult to get some contributors to link their github / w3c accounts using the OAUTH endpoint.

For these cases, it would be great if there was a different, staff-only way to link github and w3c accounts together that could be reused by other tools (esp. the repo manager) to link the identities.

[dontcallmedom]

for the W3C API, this association would have to be exposed and queryable, similarly to /users/connected/github/1234 - e.g. /users/verified/github/foobar.