[forms] closes #2437859 - Detect and prevent concurrent edition of the same entity. Add the timestamp of form generation to each entity's meta-information fields. On form validation, check that no concurrent change is overwritten and raises a ValidationError in case of concurrent change. A nicer handling with a message and a link to the new version of the entity would be a good thing...

missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/share/pyshared/cubicweb/skeleton/debian/rules.tmpl)