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
Merge pull request #7002 from olafmandel/user_avatar_in_relative_root

Fix avatar URLs in JS-lists for relative_url_root