add app3 to multitail-prod
Profiling and improving left_nav generation

* Small refactorings to leftnav(), get_full_contentsection_list() and
  lef_nav.html to try to reduce the number of SELECTs. Current runnig
  count 1200->810.
Merge branch 'master' of github.com:Stanford-Online/class2go into sef-work
cache stats should print hourly, not every two min
Fixes many left nav problems:

Mainly, puts context back so that it will hightlight where the student
is and also refactors and cleans up the code a lot.  Maintains the
ajax-loading and caching. closes #1746 and closes #1843

Also closes #1584 by putting them back in for exams / surveys / quiz
Merge pull request #1893 from Stanford-Online/sef-work

Print cache stats to django log hourly instead of every two minutes
fix div by zero error in cachestats (#1901)
Merge pull request #1902 from Stanford-Online/sef-1901-div-by-zero-in-cachestat

fix div by zero error in cachestats (#1901)
Merge pull request #1900 from Stanford-Online/jbau-jrbl-refactoring_left_nav

Jbau jrbl refactoring left nav
Merge branch 'master' of github.com:Stanford-Online/class2go into kluo-1849-section-nav-links
Get sections from common_page_data