Initial commit of new feature to show item-count within a horizontal tab

Depending on the context, we can now a show a number that lets
the user know how many of the relevant items there are.

The context could be
   1. how many quizzes are there in a student's inbox / outbox
   2. how many exams a grader still needs to grade etc. etc.

** Periodic updates ** has not been implemented. It can be, but:
  1. it will be a bit complicated - requiring regular pings to the server
  2. possibly unnecessary given that few item counts will change dramatically
     over a typical session. Our system is not like Gmail where many post
     to one account. Most of our data is slow changing
Minor corrections