Fixes #1929 uses right title for videos with quizzes
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)