Add missing relation types items in schema global variables Some of these global variables are used, in particular, to control schema views (web/views/schema.py). While they are currently not properly documented (see #3360868), their definition are currently incomplete (e.g. one could reasonably expect to find ``cw_schema`` in ``SYSTEM_RTYPES``). Closes #3486114.

[web] return 403 for Unauthorized, not 401 401 is reserved to HTTP authentication. Just because it's also called "unauthorized" doesn't mean it's the same as cubicweb's Unauthorized exception. Closes #3648809.