[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/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/