[#2577] Use CKEditorWidget for SSD help-text fields

[pi-sigma]

Taiga: https://taiga.maykinmedia.nl/project/open-inwoner/task/2577

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 44.24%. Comparing base (1d83a79) to head (80e830c).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1274      +/-   ##
===========================================
+ Coverage    44.23%   44.24%   +0.01%     
===========================================
  Files          973      973              
  Lines        35602    35610       +8     
===========================================
+ Hits         15747    15755       +8     
  Misses       19855    19855              

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[pi-sigma]

We also need to decide which formatting options we want to support, hence the update to CKEDITOR_CONFIGS in the settings. I made a stab at what seems like a reasonalbe default to me; let me know if this is too much or not enough.

[jiromaykin]

Minor style addition needed + I am quite happy users can still add headings... They cannot add an H1 anyway currently, so no problem for accessibility.