[entities] Optimize CWUser.properties.
This property gets called each time a user connects to a repository,
which means a lot.
This optimisation cuts its execution time by 5 according to my tests
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/