FEATURE: add support for group visibility level

There are 4 visibility levels

- public (default)
- members only
- staff
- owners

Note, admins and group owners ALWAYS have visibility to groups

Migration treated old "non public" as "members only"

db/migrate/20170703144855_add_visibility_level_to_groups.rb

@@ -0,0 +1,12 @@
+class AddVisibilityLevelToGroups < ActiveRecord::Migration
+  def change
+
+    add_column :groups, :visibility_level, :integer, default: 0, null: false
+    execute <<~SQL
+      UPDATE groups
+      SET visibility_level = 1
+      WHERE NOT visible
+    SQL
+    remove_column :groups, :visible
+  end
+end