[#1624] Escape html of product content field

[pi-sigma]

Taiga: #1624

[codecov-commenter]

Codecov Report

Merging #718 (4b1fa79) into develop (ceacfe5) will increase coverage by 0.09%.
Report is 90 commits behind head on develop.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #718      +/-   ##
===========================================
+ Coverage    96.20%   96.30%   +0.09%     
===========================================
  Files          642      683      +41     
  Lines        22933    24202    +1269     
===========================================
+ Hits         22063    23308    +1245     
- Misses         870      894      +24     

Files Changed	Coverage Δ
src/open_inwoner/pdc/models/product.py	90.97% <100.00%> (+0.39%)	⬆️
src/open_inwoner/pdc/tests/test_product.py	100.00% <100.00%> (ø)
src/open_inwoner/utils/ckeditor.py	88.63% <100.00%> (+0.26%)	⬆️

... and 70 files with indirect coverage changes

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[Bartvaderkin]

The double slashes come from CKeditor or its markdown plugin trying to a some sort of html escape by putting double backslash in front of the angle bracket. If I use the same javascript libraries to render the string it does escape the html-tags, so this feels like a marked/gfmarkdown thing. But I cannot find where this happens.

So I appended a commit with a slightly different approach to clean this up: we undo the weird escape and then strip_tags.

[alextreme]

@Bartvaderkin please doublecheck your solution together with the ckeditor-table option.

I've checked on production where I could find HTML-tags, and noticed that this was the case with 4 products on MG:

https://mijn.groningen.nl/onderwerpen/producten/bijstand-voor-tweedehands-computer-aanvragen/

If we strip tags I'd prefer not to remove (from the editor) valid content. I now also suspect that these tables are the reason why HTML-content is allowed...

[Bartvaderkin]

@alex I updated this PR: we no longer do html escape or strip tags, and only remove the weird slashes at render time as they are needed to be able to edit the html again (I guess the slashes are escaping html in ckeditor).

[alextreme]

Prima voor nu, graag merge conflicts verhelpen en een aparte issue maken voor ckeditor+tables