Make static pages use ContentGroup.groupinfo_by_id
Correctly show parent when editing exam

* Correctly display the parent on the exam edit dialog for child exams.
  (closes #1825)
  (closes #1914)
a bit more defensiveness in is_synced()
Fix return type of create_exam's prefill_children

* Addressing comments on PR #1918, fixing the return type of
  prefill_children and it's call signature in more places.
* Also cleans up some validation errors in create_exam.html
Fixes #1929 uses right title for videos with quizzes
Merge pull request #1928 from Stanford-Online/jbau-upload-robustness

a bit more defensiveness in is_synced()
upgrade to storages=1.1.6, boto 2.7.0 (#1923)
remove storages response_headers() patch (#1923)

We still need this patch, we are just doing via monkeypatching instead
of patching the file on disk (thanks to @jrbl).
videocheck, print id and filename to help debugging
Merge pull request #1934 from Stanford-Online/sef-1923-boto-storages-versions

Upgrade boto and storages versions (#1923) to master
section subtitles
subtitle migration
subtitles for contentsection
use dictionary.get() in cachestats (#1921)

based on feedback from @jbau in code review, small little improvement.
Merge pull request #1936 from Stanford-Online/jbau-section-sub-title

Jbau section sub title
Merge pull request #1937 from Stanford-Online/sef-1921-cachestat-followup

use dictionary.get() in cachestats (#1921)
20 minute cache leftnav
Merge pull request #1938 from Stanford-Online/jbau-20min-leftnav

20 minute cache leftnav
Merge pull request #1918 from Stanford-Online/jrbl-1825-editing_exams_keep_contentgroup

Jrbl 1825 editing exams keep contentgroup
#1945; Interactive quiz summary in-line report.