Add a system check that detects WAFs

[Findus23]

Multiple people have reported issues with tag manager caused by web application firewalls.
Many are blocking form submission when the POST data contains HTML.

I'd propose adding a system check that trie sending POST data with HTML in it and checks for a 403.

See also https://stackoverflow.com/questions/57843666/saving-custom-scripts-in-matomo-tag-manager-results-in-403/57844903#57844903
https://forum.matomo.org/t/benutzerdefiniertes-html-im-tag-manager/33535
https://forum.matomo.org/t/custom-html-not-saving/32927/5

[Findus23]

See also matomo-org/matomo#3371 for a more general mod_security detection (which might be harder to do reliably than this specific rule).

[AltamashShaikh]

Closing this as we are not fixing this in core