Use UserProfileAvatarSettingsManager from Djblets

Review Request #9952 — Created May 16, 2018 and updated — Latest diff uploaded

Information

Review Board
release-3.0.x
4d5b437...

Reviewers

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.