Add group_access field to all xblocks

TNL-670

common/lib/xmodule/xmodule/modulestore/inheritance.py

@@ -57,6 +57,15 @@ class InheritanceMixin(XBlockMixin):
         default=False,
         scope=Scope.settings,
     )
+    group_access = Dict(
+        help="A dictionary that maps which groups can be shown this block. The keys "
+             "are group configuration ids and the values are a list of group IDs. "
+             "If there is no key for a group configuration or if the list of group IDs "
+             "is empty then the block is considered visible to all. Note that this "
+             "field is ignored if the block is visible_to_staff_only.",
+        default={},
+        scope=Scope.settings,
+    )
     course_edit_method = String(
         display_name=_("Course Editor"),
         help=_("Enter the method by which this course is edited (\"XML\" or \"Studio\")."),

common/lib/xmodule/xmodule/split_test_module.py

@@ -80,10 +80,6 @@ class SplitTestFields(object):
     # location needs to actually match one of the children of this
     # Block.  (expected invariant that we'll need to test, and handle
     # authoring tools that mess this up)
-
-    # TODO: is there a way to add some validation around this, to
-    # be run on course load or in studio or ....
-
     group_id_to_child = ReferenceValueDict(
         help=_("Which child module students in a particular group_id should see"),
         scope=Scope.content