MDL-13936 indicate that user needs group membership to add new discussion, but only...

MDL-13936 indicate that user needs group membership to add new discussion, but only if having the capability to add new discussion - previously the message was incorrectly shown even when not having the cap in separate mode, visible mode needs this message too after previous commit; new lang string added in case user selects all participants in group selector and does not have accessallgroups - the existing group membership string could not be reused; merged from MOODLE_19_STABLE