Edit section and subsection at course outline page.

As a course author, I want to view and change the release date and time for subsections/sections and the due date and time and grading type for subsections from the course outline page.

https://openedx.atlassian.net/browse/STUD-1881

Test instance: http://studio.tymofij.m.sandbox.edx.org/

**VIEWING:** 

 * From the course outline, release date for sections and subsections should be viewable as per UX visual design.
 * From the course outline, due date for graded subsections and grade type should be viewable as per UX visual design.

**EDITING:**

 * _For sections and subsections_: from the course outline page, clicking the gear icon for a subsection should open a popup with fields to edit the release date and time.
  - This action should change the release date of the current subsection and all children units.
  - If the release date and time is removed, the release date and time should be updated to inherit from the parent section (replaces the current “sync” option if due dates for children differ from parent).
 * _For subsections only_: From the course outline page, clicking the gear icon for a subsection should open a popup with fields to edit the due date and time and the grading type (not graded, homework, lab, midterm exam, final exam, etc.)
  - If the due date/time is removed, the subsection should not have any due date/time. (No inheritance.)
 * You cannot delete the _section_ release date.

**NOTES:**
 * current implementation does not follow mockups visually but uses existing modal window design.
 * `staff only (overrided release date)` checkbox present in mockups is not implemented (outside of the scope of the task)
 * modal window does not look good when browser window is resized to less then 1116px


@cahrens @andy-armstrong @frrrances please review.
Please add a JS unit test for isEditable.