[RichString] Add markdown support
Supporting markdown requires the 'Markdown' python packages, and since it
widely available on the target platforms adding it as a strong requirements
will not be a big constraint.
Closes #3814302
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