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. Testing Done: Started with a fresh database and virtualenv. Saw that built-in authentication backends were available in the authentication page of the admin site.