Remove code and template responsible for subsection edit page
fixing long names on outline, unit, container
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.
Merge pull request #4635 from edx/tim/performance-get_parent_xblock

Avoid unnecessary calls to get_parent_xblock in _course_outline_json
Compute graders just once per Course.
Merge pull request #4632 from edx/tim/performance-graders-optimization

Compute graders performance optimization
Don't use `click_css` in `add_unit`.