Fixed avatar not displaying in issue boards

Phil Hughesrequested to merge
issue-boards-no-avatar into master

What does this MR do?

When gravatar is disabled, the avatar URL is returned as null. Issue boards didn't do anything about this & attempted to render an with src as null. This fixes that by specifying the no_avatar image when gravatar is disabled.

Screenshots (if relevant)

Screen_Shot_2017-04-28_at_11.29.12

Screen_Shot_2017-04-28_at_11.29.07

What are the relevant issue numbers?

Closes #31428 (closed)

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