Change quiz headings to unique level 4

Modify the "Quiz" headings:

* Make them level 4 instead of level 3. They'll still be level 3
  in edX, but making them level 4 in the markdown is logically
  more accurate and makes generated table-of-contents much
  easier to understand.
* Make section headings unique by adding ": name-of-parent"
  for each quiz heading. Anchor names are generated from section names,
  and it would be a problem if hyperlink tried to go to a dupllicate
  section name. We want to turn on markdownlint's check for
  unique section names to prevent this, but we can't as long as
  quiz section names duplicate. So let's eliminate the duplication.
  I kept the quiz numbers (per request from Flavia) to simplify
  working with edX. Note that "Quiz: Avoid Integer Overflow,
  Wraparound, and Underflow" does *not* have a quiz number; that's
  odd, but not a problem because the text after the colon makes
  the heading unique.

Signed-off-by: David A. Wheeler <[email protected]>

Author: david-a-wheeler