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.

+
−import ConfigParser
+
−try:
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−    undo_actions = config.cfgfile_parser.get('MAIN', 'undo-support', False)
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−except ConfigParser.NoOptionError:
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−    pass # this conf. file was probably already migrated
+
−else:
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−    config.global_set_option('undo-enabled', bool(undo_actions))