Changes to get it working on Redmine 2.6.0

I have this plugin mostly working on 2.6.0, although I had to hack out a few bits that I couldn't get to work.  The main ugliness is that the users-to-add list is initially empty until you type some name in the list to chose users to allow to be group assignees.  In particular I moved the autocomplete_for_assigned bits around different places trying to get it visible in the page template, and finally gave up.