Fix delete messages in course outline
STUD-2019
Reduce calls to expensive methods in create_xblock_info

Special-case the outline page requests.

STUD-1997
Merge pull request #4604 from edx/dan-f/improve_outline_performance-2

Reduce calls to expensive methods in create_xblock_info
Merge pull request #4601 from edx/benmcmorran/delete-message

Fix delete messages in course outline
Reduce parent/child lookups in create_xblock_info

STUD-1997
Merge pull request #4614 from edx/dan-f/improve_outline_performance-2

Further performance improvements to course outline
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