Git commits in Moodle 3.6.0 by David Balch

commit 528060a1a968b640447eac128030a8ba60816dd6
Author: David Balch <david.balch@conted.ox.ac.uk>
Date:   Mon, 28 May 2018 12:42:16 +0800
Tag:    v3.6.0-beta

MDL-57742 core_lib: Make tool_consumer_instance_guid optional