Fix a couple problems with the related user widget and avatars.

Review Request #1200 — Created May 2, 2018 and updated — Latest diff uploaded

Review Board
The related-user widget had two problems when dealing with avatars: one
on the server side, and one on the client.

On the server, if no avatar services were available for a user, we'd get
an uncaught `AttributeError` when trying to get the
`get_avatar_urls_uncached` method on a `NoneType`. On the client, once
we'd discovered a user without any avatars, we'd neglect to add a blank
`avatarURL` field to the data for the widget, resulting in a JavaScript

Testing done:
Set the default avatar service to "None" and added an avatar-less user
to a group. Loaded that group in the admin UI and saw that I could see
the page, including the users widget, without any Python or JavaScript

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