FIX: videosequence was not inheriting metadata, eg start date

For a module to be able to inherit metadata from its parent, it needs to be included in a list in `common/lib/xmodule/xmodule/modulestore/mongo/base.py`.  The list left out `videosequence` by mistake.

This was breaking a number of MIT courses, eg 2.01x.  

A symptom of this error is when "start" dates are not being inherited by problems inside videosequence containers; they show up as the default 1970 instead.