feat: Add Sidebar and SidebarNotificationButton components for Value Prop

[julianajlk]

This is a duplicate of https://github.com/edx/frontend-app-learning/pull/390, which has unknown build failures. This PR is the same but commit 68cf1db is divided up into separate commits here. More commits were added after commit 3bf88c7 Styling/a11y updates.

REV-2125.

For Value Prop implementation, we will implement a sidebar that is triggered on click of a button with upgrade messaging and CTA. Pls note this ticket does not cover adding the upgrade messaging and CTA, it only covers adding the button trigger and sidebar component.

Large screen size (> 992px):

Responsive at medium screen sizes (> 576px and < 992px): it has the button like large and above screen size, but the sidebar behaves in full width mode like for small screens.

Responsive at small screen sizes (< 576px):

[codecov]

Codecov Report

Merging #414 (bc2e75f) into master (3e82152) will increase coverage by 1.24%.
The diff coverage is 91.48%.

@@            Coverage Diff             @@
##           master     #414      +/-   ##
==========================================
+ Coverage   82.01%   83.26%   +1.24%     
==========================================
  Files         190      201      +11     
  Lines        3031     3442     +411     
  Branches      753      887     +134     
==========================================
+ Hits         2486     2866     +380     
- Misses        529      556      +27     
- Partials       16       20       +4     

Impacted Files	Coverage Δ
src/courseware/course/CourseBreadcrumbs.jsx	95.45% <ø> (ø)
src/courseware/course/NotificationIcon.jsx	66.66% <66.66%> (ø)
src/courseware/course/sequence/Sequence.jsx	89.65% <85.71%> (-0.35%)	⬇️
src/courseware/course/Course.jsx	96.96% <90.00%> (-3.04%)	⬇️
src/courseware/course/Sidebar.jsx	100.00% <100.00%> (ø)
...rc/courseware/course/SidebarNotificationButton.jsx	100.00% <100.00%> (ø)
src/courseware/course/messages.js	100.00% <100.00%> (ø)
...equence/sequence-navigation/SequenceNavigation.jsx	100.00% <100.00%> (ø)
src/generic/tabs/useWindowSize.js	86.66% <100.00%> (+0.95%)	⬆️
...ss-tab/grades/grade-summary/GradeSummaryHeader.jsx	87.50% <0.00%> (-12.50%)	⬇️
... and 24 more

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[dianekaplan]

No issues jump out at me- though I'm less familiar with this stuff so I'm not sure if they would.

One question would be around the unit test expectations: I see a couple lines flagged that aren't covered by unit tests yet. I that something that'll be included in this PR, or is it done more on a case-by-case basis for frontend-app-learning?

[julianajlk]

@dianekaplan good question, something I wonder too - overall test coverage went up with this PR and it does meet the minimum for merging, but not sure where to draw the line with UI tests to be honest. I can add some more if that's going to make the code more reliable

[jhan217]

Looks fine to me, couple of minor comments