fix: breadcrumb and sidebar preview link

[KristinAoki]

Description

This PR fixes the left sidebar navigation and breadcrumbs not having preview links when a staff user is viewing a course in "Preview" mode.

Supporting Information

GitHub Issue #1657

Testing

Breadcrumbs

1. In Django Admin disable courseware.enable_navigation_sidebar
2. Open a course in Studio
3. Navigate to a unit
4. Make a change to confirm that you are seeing a preview
5. Click "Preview"
6. Click on the section or subsection breadcrumb
7. Confirm that the url includes /preview/
8. Confirm that the draft version of the unit is still visible

Left sidebar navigation

1. In Django Admin enable courseware.enable_navigation_sidebar
2. Open a course in Studio
3. Navigate to a unit
4. Make a change to confirm that you are seeing a preview
5. Click "Preview"
6. Click on the highlighted unit
7. Confirm that the url includes /preview/
8. Confirm that the draft version of the unit is still visible

[openedx-webhooks]

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[codecov]

Codecov Report

Attention: Patch coverage is 91.42857% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.06%. Comparing base (d5a092b) to head (c1e3bca).

Files with missing lines	Patch %	Lines
...c/courseware/course/breadcrumbs/BreadcrumbItem.tsx	88.88%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1660      +/-   ##
==========================================
- Coverage   90.09%   90.06%   -0.04%     
==========================================
  Files         338      339       +1     
  Lines        5784     5795      +11     
  Branches     1370     1412      +42     
==========================================
+ Hits         5211     5219       +8     
- Misses        557      558       +1     
- Partials       16       18       +2     

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