user selection: MDL-16993 Convert the assign roles page to use the new components.

user selection: MDL-16993 Convert the assign roles page to use the new components.

At the same time, I took the opportunity to try to work on some of the usability issues on this page. Note that I have not quite finished! So don't comment until tomorrow.

In the course of doing this, I also did: half of

MDL-11529 Show the number of assignees of each role in the change role dropdown on this page.
MDL-17067 Make it clear in the UI that admins are not allowed to unassign themselves.