Added course level lang attribute for LMS

added lang attribute to Studio

fixed error when trying to get language attribute on course that does not exist

simplified code

added lang attribure to more places in LMS, added migration for language attribute in course_overview

fixing contraints in the database for language attribute

added lang attribute to several more places in LMS and Studio

added lang attribute to discussion and custom pages; cleaned up code

fixed issue that was causing test failures

moved lang attribute higher in tree in two places