Fix RelatedUserWidget regressions from recent avatar changes.

Review Request #1688 — Created July 3, 2019 and updated

Review Board
The `RelatedUserWidget` HTML includes information on user avatars for
rendering purposes. Recently, Djblets avatar rendering was updated to
provide HTML instead of just URLs for this widget, in order to support
the fallback avatar service. This regressed some tests we had only on
Review Board 4.0.

This change updates the HTML comparison in a test for the new rendering.

Testing Done:
Unit tests pass against Django 1.6 and 1.11.

Reviewed at