Entry points are fantastic, and they're a great way for us to allow

third parties to integrate their own plugins with Review Board.

Unfortunately, if there's a VersionConflict present in the install, no

entry points will load, even if things will otherwise work. This means

that no alternative authentication backends will load, which is extra

bad because this may make it particulary difficult for administrators to

log in to even start debugging the issue (such as looking at the log

viewer in the admin).
This change moves the built-in authentication backends into the

registry's get_defaults() method, rather than using entry points. Any

third-party services will still be loaded from entry points afterwards.

Started with a fresh database and virtualenv. Saw that built-in

authentication backends were available in the authentication page of the

admin site.