feat: Added raw loader for TinyMCE conten.css import

[PKulkoRaccoonGang]

Description

Added ?raw loader to TinyMCE conten.css import.

Additional info

• fix: Remove extra margin around body element #193
• https://stackoverflow.com/questions/70642036/tinymce-not-loaded-inside-the-iframe-chrome-extension-tinymce-should-have-bee

Quick demo

How Has This Been Tested?

1. Open the page: http://apps.local.openedx.io:1984/communications/courses/course-v1:OpenedX+DemoX+DemoCourse/bulk_email
2. The TinyMCE editor should look the same as it did before adding the ?raw loader, with no visual issues.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.14%. Comparing base (7ddc950) to head (599cc43).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #260   +/-   ##
=======================================
  Coverage   84.14%   84.14%           
=======================================
  Files          47       47           
  Lines         700      700           
  Branches      135      136    +1     
=======================================
  Hits          589      589           
  Misses        111      111           

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