clean up imports
start new home page

Goal is to make some room for progress tracking UI features.
working on the exercises tab of CoursesAndProblemsPage3

other misc. cleanups, including a bugfix in ExerciseSummaryView -
a ProblemAndSubmissionReceipt may not have a SubmissionReceipt
(if the user has never worked on the Problem)

tweaks to the CSS for exercise summary items (make the boxes
smaller, and decrease the about of space between them)
implemented refresh and load exercise buttons
implemented account tab in CoursesAndProblemsPage3

Also, fixed bug in CourseSelectionListBox - need to clear
listbox when a list of CourseAndCourseRegistrations is loaded
implement "manage exercises" and "manage users" buttons
implemented playground tab of CoursesAndProblemsPage3

Also, made it the default, since all of the functionality of
CoursesAndProblemsPage2 is now restored.
a few visual tweaks
display tooltips on ExerciseSummaryItems
make ExerciseSummaryItems clickable
allow ProblemListView2 to respond to change to selected Problem

This allows the exercise selected in, e.g., the ExerciseSummaryView,
to be reflected in the problem list.
tweaks to style of exercise summary items (make colors lighter)
add some horizontal padding to the exercise description

This has been bugging me for years - why didn't I fix it
a long time ago? :-)
implemented logout handler in CoursesAndProblemsPage3
new simplified problem list that displays best score bars

Changed red/green colors in score bars to match the softer
red/green colors used in the exercise summary view.