factored out EditUserView from Andrei's add user popup

Goals are

- to reuse for for adding and editing users
- to support flexible client-side validation
- to allow section to be entered

Added an EditedUser model object, since a User object alone
doesn't enough info on its own.  Users will typically be
edited in the context of a course, where details such as
registration type and section are important.
switched to new add user dialog and RPC/database back end

Also, Fixed bug where the RPC servlet and database weren't verifying
that the authenticated user was an instructor in the course in which
the student was being added.
purge Problem/QuizInProgress from session when leaving dev page

This avoids problems where a Problem that the user should
not be allowed to see/use (such as a finished quiz) is left
in the client-side session.

Also, prevent the use of the "Reset" button if edits are
disallowed (such as when a quiz has ended).
Merge pull request #34 from daveho/master

improvements to quiz mode