Merge branch 'fix-default-avatar-in-network-graph' into 'master'

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

See merge request !1411