Default type instance for AuthEntity

`AuthEntity` is now a type family associated to the `YesodAuthPersist`
class, so its old type alias definition has now become its default type
instance, which allows recovering the old behavior with minimal
boilerplate: an empty instance declaration for `YesodAuthPersist` makes
a Yesod application written with Persistent work just like before.

Unfortunately, this requires enabling `UndecidableInstances` in the
`Yesod.Auth` module since there is now a nested type family application
(as `AuthEntity` is now itself a type family).

This was tested with the scaffolded application with PostgreSQL, and it
works (given another small change to make it work with Persistent 2 due
to how `SqlPersistT` is now defined).

yesod-auth/Yesod/Auth.hs

@@ -9,6 +9,7 @@
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE DeriveDataTypeable #-}
+{-# LANGUAGE UndecidableInstances #-}
 {-# OPTIONS_GHC -fno-warn-orphans #-}
 module Yesod.Auth
     ( -- * Subsite
@@ -392,6 +393,7 @@ class (YesodAuth master, YesodPersist master) => YesodAuthPersist master where
     --
     -- Since 1.2.0
     type AuthEntity master :: *
+    type instance AuthEntity master = KeyEntity (AuthId master)
 
     getAuthEntity :: AuthId master -> HandlerT master IO (Maybe (AuthEntity master))