Fix width in popups with the richEditor mounted

[igraziatto]

Closes https://github.com/vanilla/support/issues/1257

Steps to test:

• You can have keystone or any other theme enabled, go to a discussion and click on "Flag" and then "Warn"
• A full width popup will display, with the richEditor mounted.
• Checkout this branch and repeat the steps above, the popup should have a max of 600px on large screens and full width on mobile.

[codecov]

Codecov Report

Merging #10289 into master will not change coverage by %.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master   #10289   +/-   ##
=========================================
  Coverage     53.35%   53.35%           
  Complexity     7846     7846           
=========================================
  Files           363      363           
  Lines         43964    43964           
=========================================
  Hits          23457    23457           
  Misses        20507    20507           

[nervoustwit]

Tested locally, looks good.