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.

db/migrate/20190812141433_add_members_visibility_level_to_groups.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddMembersVisibilityLevelToGroups < ActiveRecord::Migration[5.2]
+  def change
+    add_column :groups, :members_visibility_level, :integer, default: 0, null: false
+  end
+end