fix: update outline sidebar hooks for plugins

[KristinAoki]

This PR moves all the useSelector and dispatch calls for the outline sidebar to outline-sidebar/hooks.jsx and moves Link into a separate component for the SidebarUnits.jsx. These changes are necessary to fully support plugins. When using any of those functions inside of a plugin it causes conflicts with redux and the router.

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[codecov]

Codecov Report

Attention: Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.61%. Comparing base (9dc45e1) to head (41c0173).

Files with missing lines	Patch %	Lines
...e/course/sidebar/sidebars/course-outline/hooks.jsx	95.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1586      +/-   ##
==========================================
+ Coverage   84.60%   84.61%   +0.01%     
==========================================
  Files         331      332       +1     
  Lines        5656     5661       +5     
  Branches     1357     1394      +37     
==========================================
+ Hits         4785     4790       +5     
+ Misses        854      852       -2     
- Partials       17       19       +2     

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[KristinAoki]

@openedx/committers-frontend-app-learning this PR is ready for review

[bradenmacdonald]

I'll try to take a look by tomorrow.

[bradenmacdonald]

I'm not sure I totally understand the necessity of this, but the refactor looks reasonable to me. Thanks!

[bradenmacdonald]

I think it would be useful to have a JSDoc string here explaining why we have this component.