cubicweb #5534074 Cleanup cubicweb:target hack on form iframes [validation pending]

This is a follow up to ticket #5377672.

The iframe hack (on the JS side) was necessary back when CubicWeb was serving HTML with an XHTML 1.1 strict doctype, which forbids iframes. Now that we're serving HTML content with an HTML5 doctype, we can simplify things and generate server-side the correct HTML form content with the iframe and its target attribute.

priorityminor
typeenhancement
done in3.21.0
load1.000
load left0.000
closed by#8e04ab5582d9 [web/views/formrenderer] do not use `cubicweb:target` attribute on form (closes #5534074)
patch[web/views/formrenderer] do not use `cubicweb:target` attribute on form (closes #5377672) [applied]