Caching the complicated pieces of common_page_data

* left_nav generation isn't the real problem, the real problem is
  common_page_data. So now we address that directly, and add some caching to
  get_common_page_data for the complicated pieces.
* Current running count of SELECTs when loading left_nav.html 810->6.
Cleanups to leftnav refactorings and related code

* Addressing @jbau's comments on #1898:
  * Commenting out calls to CacheStat pending resolution of #1905
  * Using draft/ready in cache keys to help eliminate potential
    information leakage
  * Elimination of view_mode (nobody seemed to be using it)
  * Elimination of current_datetime and effective_current_datetime (these
    were also unused)
  * Careful testing of the appearance of leftnav, behaviour of static
    pages and exam creation