Use UserProfileAvatarSettingsManager from Djblets
Review Request #9952 — Created May 16, 2018 and updated
The UserProfileAvatarSettingsManager has been migrated to Djblets as of
https://reviews.reviewboard.org/r/9951/, so there is no reason for us to
continue to include it in Review Board.
Ran unit tests.
This is in the wrong import group.