cubicweb-vcreview #1655131 make it possible to comment patchs like in steve losh's hg-review [open]

Steve Losh is writing a hg extension hg-review in which one can comment any line of a commit. We should have a similar way of commenting a patch.

hg-review: https://bitbucket.org/sjl/hg-review/src

prioritynormal
typeenhancement
done in<not specified>
closed by<not specified>