Fix regressions from recent authentication backend cleanup.

Review Request #1317 — Created July 16, 2018 and updated — Latest diff uploaded

Review Board
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

    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