Fix default avatar not showing up in network graph

The change in 71cdb249 removed the /assets path in the default avatar to eliminate runtime errors, but this was one place where image_path was missed. If the user full URL is specified, image_path does nothing.

Closes https://github.com/gitlabhq/gitlabhq/issues/9657

mentioned in issue #2572 (closed)

Added 74 commits:

• caf500c6...c4ca1ad4 - 73 commits from branch gitlab-org:master
• 45262120 - Fix default avatar not showing up in network graph

Status changed to merged

mentioned in commit ede1ee85

Picked into 8-0-stable.