feat: add page not found route

[KristinAoki]

Description

This PR replace the blank screen for invalid URLs with a header, footer, and page not found message. The page not found page also sends a logging error and tracking event so that redirects to invalid urls can be monitored and tracked.

Before

After

Supporting Information

JIRA Ticket: AU-1049 🔒

Due to MFEs' current client-side routing implementation, users just saw a blank page with a footer (ie, a “client-side 404”) instead of a true 404 page. The routing failures yielded a 200 OK page and are not reported to the server in any way, so no alerts are triggered. For contrast, 404s in Legacy LMS pages are logged and also share a more coherent story with users.

Testing

1. Navigate to a course
2. Change the URL to an invalid path
3. Confirm that the PageNotFound component is displayed
4. Click the homepage URL
5. Confirm that it navigates back to the learner dashboard

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 89.14%. Comparing base (6534347) to head (2a29602).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/generic/PageNotFound.jsx	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1514      +/-   ##
==========================================
- Coverage   89.23%   89.14%   -0.10%     
==========================================
  Files         317      318       +1     
  Lines        5557     5563       +6     
  Branches     1375     1379       +4     
==========================================
  Hits         4959     4959              
- Misses        582      588       +6     
  Partials       16       16              

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