Drag and drop support on the course outline page.
Studio: re-implemeting visual styling for drag and drop:

* adding back in drag and drop indicators to outline DOM
* correcting visual display of drag action in outline UI
* adjusting styling of outline drag and drop indicators
* adjusting styling of drag and drop was-dropped state
resolving mid-drop unit marign issue in Firefox
Increase "fudge" factor for dragging to top of list with only 1 element.
Remove code and template responsible for subsection edit page
styling for unit location module on container page
fixing long names on outline, unit, container
Merge pull request #4605 from edx/christina/drag-drop-outline-3

Christina/drag drop outline 3
Merge pull request #4627 from edx/frances/feature/unit-location-bp

Unit Location on Unit page styling
Merge pull request #4631 from edx/frances/fix/unit-name-wrap

fixing long names on outline, unit, container
Merge pull request #4540 from edx/tim/delete-all-the-subsections

Delete standalone Subsection edit page
Pass parent_xblock to xblock_studio_url to avoid unnecessary calls to get_parent_xblock.