get_group_tags lists the tags under one group category. add_group_tag
and delete_group_tag add or remove a tag by name. Callers pass the
category number as group_class. An empty name is rejected.
Refs #506
Bots can now read the current join option, robot-add option, member
invite policy, member permissions, history visibility, and search
settings that previously only had setters.
Fixes#385