Verify grade export works for problems in split tests

TNL-41
Merge pull request #5728 from edx/dan-f/grade-export-split-test

Verify grade export works for problems in split tests
Studio: visually styling current unit on course unit/container view's mini outline
Studio: correcting minor Sass indent and property syntax issues
Show current unit in unit outline.

TNL-291
Fix i18n and quality errors in course_groups/views.py
generalize draft import/export TNL-574

add test for videomodule

Import drafts handles modules w/o xml_attributes
and ones using the old parent_sequential_url pointer

Handle drafts for modules which don't define nor extract xml_attributes

Support  and  in get_items qualifiers

Unit test for   value query
Corrected link to Developer's guide

@mhoeber , @stroilova
Merge pull request #5769 from edx/ahodges/doc/DOC491

Corrected link to Developer's guide
Merge pull request #5792 from edx/sarina/i18n

Fix i18n and quality errors in course_groups/views.py
Merge pull request #5764 from edx/dan-f/highlight-active-unit-in-outline

Highlight active unit in studio container page's outline
Merge pull request #5538 from edx/adam/generalized-draft-import-export-2

Adam/generalized draft import export 2