MDL-79581 communication: Context is a mandatory field

The API was incorrectly assuming that all uses of the API were for a
course, and that the instanceid of the communication instance was a
course id. These assumptions are both entirely wrong.

The API is intended to support a range of uses including use at the
site, user, and activity levels.

Furthermore, if a group were to be used, then the instanceid should be
of that group's id, and therefore the contextid would need to be fetched
or that group's course instead.

The only solution here is to add a new contextid field to the table, and
implement it all parts of the API.

lib/accesslib.php

@@ -1648,6 +1648,7 @@ function role_assign($roleid, $userid, $contextid, $component = '', $itemid = 0,
     // Update the room membership and power levels when the user role changes.
     if (\core_communication\api::is_available() && $coursecontext = $context->get_course_context(false)) {
         $communication = \core_communication\api::load_by_instance(
+            $coursecontext,
             'core_course',
             'coursecommunication',
             $coursecontext->instanceid,
@@ -1765,6 +1766,7 @@ function role_unassign_all(array $params, $subcontexts = false, $includemanual =
             // Update the room membership and power levels when the user role changes.
             if (\core_communication\api::is_available() && $coursecontext = $context->get_course_context(false)) {
                 $communication = \core_communication\api::load_by_instance(
+                    $coursecontext,
                     'core_course',
                     'coursecommunication',
                     $coursecontext->instanceid,