Delete standalone Subsection edit page

https://openedx.atlassian.net/browse/STUD-1845


  *  User should not be able to access the Subsection page from anywhere in Studio.
  *  Mini-outline should no longer link to the Subsection page.
  *  Breadcrumb should not link to the Subsection page.
  *  Click on the subsection name on the outline page to edit subsection name.
  *  Update tests, delete all the code.

Technical details:

 * Remove `overview.js` and `overview_spec.coffee`
 * Remove `base.js` (or at least as much of it as possible)
 * The Lettuce tests should already be gone, but double check
 * There were no Bok Choy tests

Implementation question:

_All_ inplace edits were changed to mode "click the text to start editing", as I did not observe this behavior in bulk-publishing branch. Tests pass, but I would like to hear your word on it.

@auraz @polesye please take a look
Please remove unused after deletion modules: `ModuleStoreEnum`, `Scope`, `CourseGradingModel`.