GitHub has recently deprecated some aspects of their Authorizations API

and authentication methods, and one of those deprecations became very

public today. Review Board has historically provided the user's access

token in the URL, which is now deprecated and triggering e-mails from

GitHub warning about this deprecation.
The correct way forward is to use the Authorization header to provide

this token. This change updates all our GitHub support to do this, and

to make note of a few places where upcoming deprecations will be

affecting us.

Tested all file retrieval operations (file existence checks, file fetching)

and repository browsing (fetching of branches and commits) by way of viewing

diffs and performing operations through the New Review Request page.
Tested linking a new account, with and without a registered GitHub OAuth

app.
Tested editing account credentials, with and without a registered GitHub

OAuth app.
Unit tests pass.