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.
Merge branch 'jrbl-refactoring_left_nav' of https://github.com/Stanford-Online/class2go into jbau-jrbl-refactoring_left_nav