Clean up of modulestore references (primarily LMS).

This pull request is to ensure that LMS is using mixed modulestore and is not depending on "direct" vs. "default". At the same time, I removed some crufty mongo configurations, and changed how some places in the code determine if a course is "in Studio" so that they will work when split is introduced.

Note that I was not able to remove the optional "modulestore name" argument to editable_modulestore. That was my goal, but it is still needed by one place-- the CourseFactory used for creating content for unit tests. When running CMS tests, it is necessary to pass in 'direct' as the 'default' is the DraftModuleStore (for LMS, 'direct' is set to map to 'default'). We will have to come up with a solution when switching to split.

Please review, @dmitchell and @sarina (or defer to another LMS reviewer)