FEATURE: add support for group members visibility level (#8004)

There are 5 visibility levels (similar to group visibility)

public (default)
logged-in users
members only
staff
owners

Admins & group owners always have visibility to group members.

test/javascripts/fixtures/group-fixtures.js.es6

@@ -46,7 +46,8 @@ export default {
       flair_url: "fa-adjust",
       is_group_owner: true,
       mentionable: true,
-      messageable: true
+      messageable: true,
+      can_see_members: true
     },
     extras: {
       visible_group_names: ["discourse"]