Fix avatar URLs in JS-lists for relative_url_root

In user lists created when entering a (partial) user name into a field, the URL to the user avatar was invalid if running with relative_url_root.

This patch is the result of:
`sed -i 's/\(= *\)\(user\.avatar_url\)/\1gon.relative_url_root + \2/' app/assets/javascripts/*.coffee`

Did not update the `CHANGELOG`, as this is after the merge window and the version number would be wrong.