cubicweb-blog #2093040 Limit width of text so it is readable [validation pending]

Often on big screens content in cubicweb is not readable.

I believe we should limit the size of main text areas.

See : http://baymard.com/blog/line-length-readability

prioritynormal
typeenhancement
done in1.9.4
load0.500
load left0.000
closed by#3ff01e768daa Add div to tweak only blog content & limit width using css (closes #2093040)