X509AuthMiddleware is an old piece of code designed to authenticate

users using X.509 certificates. It was living in

reviewboard.admin.middleware, rather than

reviewboard.accounts.middleware, had some unnecessary checks, and

determined whether to process based on logic that the account backend

split is going to break. That last reason is why this change is being

made.
This change fixes all that up, simplifying the code, moving it under

reviewboard.accounts, and determining the auth backend through safer

means. It also adds unit tests to ensure that authentication only

happens under the proper circumstances.

Unit tests pass.