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/Handler/Entries.hs

@@ -9,6 +9,7 @@ import Yesod.AtomFeed ( atomLink )
 import qualified Data.Text as T
 
 import qualified Entry.Kind as K
+import Model.Cache ( getCached )
 import Widget.Entry ( entryR, hEntry )
 import Widget.Feed ( hFeed )
 
@@ -22,13 +23,13 @@ getEntriesR kind = do
         $(widgetFile "entries")
 
 getEntryR :: a -> EntryId -> Handler Html
-getEntryR _ entryId = getEntry <=< fmap (Entity entryId) . runDB . get404 $ entryId
+getEntryR _ = renderEntry <=< getCached
 
 getEntryWithSlugR :: a -> EntryId -> b -> Handler Html
 getEntryWithSlugR kind = const . getEntryR kind
 
-getEntry :: (Entity Entry) -> Handler Html
-getEntry entry = do
+renderEntry :: (Entity Entry) -> Handler Html
+renderEntry entry = do
     let correctRoute = entryR entry
     actualRoute <- getCurrentRoute
     when (actualRoute /= Just correctRoute) $