Split distribution table and ratings table

HTML for the distribution and ratings table can now be generatad
seperately.
Redo teacher controls

Removed clear distribution button and timeout field. start_form is no
longer used.
Split the show table button and start distribution button into
separate functions.
The distribution table is now always visible.
The show table button shows the ratings table and disappears
afterwards.
Delete start_form.php

This file is no longer used.