fix: library editor not working locally

[jesperhodge]

Internal issue: https://2u-internal.atlassian.net/browse/TNL-11299

In library-authoring, content blocks could not be edited locally due to some problem with the Accept header. This caused some 404s. Visually you would get an infinite loading spinner.

This depends on openedx-unsupported/frontend-app-library-authoring#399.

Testing:

• add MFE_NAME='frontend-app-library-authoring' to .env.development in your local frontend-app-library-authoring. (I'll add an extra PR for that now as well).
• test that your local course-authoring opens text blocks and that you can edit and save them. (In a previous attempt to fix this, the change broke course-authoring.)
• test that your local library-authoring opens text blocks and that you can edit and save them.
• When code reviewing, pay attention that the new code should not affect stage or prod at all and only touch local behavior.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6945234) 90.57% compared to head (61f9250) 90.58%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #448   +/-   ##
=======================================
  Coverage   90.57%   90.58%           
=======================================
  Files         227      227           
  Lines        4129     4132    +3     
  Branches      831      832    +1     
=======================================
+ Hits         3740     3743    +3     
  Misses        369      369           
  Partials       20       20           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.