[WIP] Add an extension for OpenID-based authentication.
This introduces OpenID-based authentication to Review Board 2.0. It

extends the login page to provide options for logging in using common

services, such as GitHub or Google. It accepts any OpenID provider.
This is still a work-in-progress. It needs the in-development

Review Board 2.0 beta 2 changes, and doesn't yet provide anything for

the My Account page. There's still documentation missing, some debug

output, and more testing that needs to be done.
This is heavily based on the work done by Patrick Uiterwijk.

Tested logging in with different services.