FEATURE: Add group settngs to allow users to leave a group freely.

https://meta.discourse.org/t/split-join-leave-freely-setting-on-groups/65565

db/migrate/20170728012754_split_public_in_groups.rb

@@ -0,0 +1,17 @@
+class SplitPublicInGroups < ActiveRecord::Migration
+  def up
+    add_column :groups, :public_exit, :boolean, default: false, null: false
+    add_column :groups, :public_admission, :boolean, default: false, null: false
+
+    ActiveRecord::Base.exec_sql <<~SQL
+    UPDATE groups
+    SET public_exit = true, public_admission = true
+    WHERE public = true
+    SQL
+  end
+
+  def down
+    remove_column :groups, :public_exit
+    remove_column :groups, :public_admission
+  end
+end