Added Quiz model object class

Also improved database migration code so that it can handle a
new table being added to the database.
when check course registrations for user/course: allow multiple results

This can happen, for example, if a user is registered as an instructor
for multiple course sections.

Added CourseRegistrationList model class to represent list of
course registrations.
user can see a problem if there is an ongoing quiz

Changed getProblemAndSubscriptionReceiptsInCourse() so that
the database query returns precisely the problems the user is
permitted to see along with their corresponding submission
receipts (if any).
use symbolic references for db table and field names
go back to hard-coded column names

The symbolic references add too much noise, making the query
very hard to read.
when getting problems in course, join course regs with user id

The previous version of the query didn't do this, so you would
effectively get problems for ALL users.
allow user to work on problem if a quiz is ongoing
add javadoc for getProblem(User, int) method
when retrieving problem for user, return quiz too if appropriate

Also, store the quiz (if any) in the user's session.
don't allow edits to problem if quiz has ended
de-dup courses in TermAndCourseTreeView if user has multiple regs

A user can have multiple registrations for the same course:
for example, an instructor who is teaching multiple sections will
have one registration per section.
cleanup, add copyright notice / class header comment
started UI for administering a quiz

Also, fixed bug - course admin page wasn't displaying the problems
because the CourseAdminProblemListView wasn't adding the problems
it was loading to the session.
fix navigation from QuizPage back to CoursesAndProblemsPage2
add label/combox box to choose section in QuizPage UI
added button to start quiz
added/implemented RPC service to start a quiz
added "End Quiz" page to QuizPage UI
started quiz timer UI, delete old quiz before starting new
QuizPage: check for current quiz and resume if there is one