Fix regressions from recent authentication backend cleanup.
Review Request #1317 — Created July 16, 2018 and updated
There were two issues found with the recent changes meant to split up and document the authentication backends. 1) `BaseAuthBackend.get_user()` was accidentally removed with commit a12a370fd. This function is required by Django to fetch a user for a given database ID. 2) The `ldap` backend's module was being imported by the Active Directory backend instead of the top-level `ldap` module. This required importing `absolute_import` to ensure the local module wasn't used. Testing Done: * Created Docker container form easybe/alpine-reviewboard:wip * Successfully logged in with Active Directory credentials Reviewed at https://reviews.reviewboard.org/r/10045/