Add per-request entity caching support

Different areas of the app need access to entities - for example both
the entry handler needs the entry itself to render it, but the
breadcrumbs also need the entry to decide what to label its breadcrumb.
Previously this was achieved by fetching entities from the database
twice. This sucks, so now it's implemented by fetching entities once and
caching them using Yesod's per-request cache.

src/Model/Cache.hs

@@ -0,0 +1,23 @@
+{-# LANGUAGE DeriveDataTypeable #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE TypeFamilies #-}
+module Model.Cache ( getCached ) where
+
+import Data.Typeable ( Typeable )
+import Database.Persist ( Entity (..), Key (..), PersistStore, PersistRecordBackend, keyToValues )
+import Yesod ( MonadHandler, HandlerSite, YesodPersist, YesodPersistBackend, cachedBy, get404, liftHandlerT, runDB )
+
+import qualified Data.ByteString.Char8 as C
+
+newtype CachedEntity t = CachedEntity { unCachedEntity :: Entity t } deriving Typeable
+
+getCached :: ( MonadHandler m
+             , YesodPersist (HandlerSite m)
+             , PersistStore (YesodPersistBackend (HandlerSite m))
+             , PersistRecordBackend entity (YesodPersistBackend (HandlerSite m))
+             , Typeable entity
+             ) => Key entity -> m (Entity entity)
+getCached entId = liftHandlerT . cached . runDB . withId . get404 $ entId
+    where key = C.pack . show . keyToValues $ entId
+          withId = fmap $ Entity entId
+          cached = fmap unCachedEntity . cachedBy key . fmap CachedEntity