[views/dot] use an inlined svg for schema and workflow graphs These are currently served as "temporary" pngs which are actually consummed immediately. Which means they cannot be requested twice and any further attempt will yield an error in the logs and some end-user surprise. There is no known acceptable workaround for IE-8 (and previous versions). SVGWEB could be workable but it's not trivial to integrate it properly. Closes #3400448.

usr/lib/PY_VERSION/*-packages/cubicweb/entities/
usr/lib/PY_VERSION/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/PY_VERSION/*-packages/cubicweb/*.py