[view] return HTML5 doctype. Closes #2869426
We dropped xhtml support in 3.17 but we kept providing documents using the
XHTML strict doctype, while our content wasn't conform to the DTD since we
dropped proper namespaces definition as well as declaration of cubicweb DTD
extensions.
[testlib] repreprocess content so contextual display has a chance to show the proper line. Closes #2869481
the context is calculated using position information relative to a preprocessed content,
not the original source