17724 respect group assignment condition

https://community.openproject.org/work_packages/17724
Hm, only noticing it now, sorry for that, but this all could be rewritten like the following:

<pre>
def principal_visible?(user_id, list)
  list.exists?(user_id)
end
</pre>

This will result in only 1 DB query as compared to 2 and it will be correct because the possible_assignees/possible_responsibles already handle the setting.