* feat: Course unit - add new component section
* feat: Course unit - make Discussion and Drag-and-Drop button functional
* feat: Course unit - make Problem button functional
* feat: Unit page - make Video button functional
Also refactor api and hooks
fix: publish button behaviour and card header tests
fix: warning in highlights and publish modal test
fix: courseoutline tests
test: add test for new section functionality
fix(lint): lint issues
refactor: remove unnecessary css in CardHeader
refactor: rename emptyPlaceholder test file
refactor: replace ternary operator with 'and' condition
refactor: add black color to expand/collapse button
refactor: display only changed subsection and units in publish modal
refactor: update messages and css
refactor: wrap urls in function call
refactor: fix jsdoc types
refactor: use helmet for document title
