Support building with yesod 1.4.

The new yesod needs the ViewPatterns extension.
Also, a TH splice in Assistant/Threads/WebApp.hs failed to work without
OverLoadedStrings.

This commit was sponsored by Brock Spratlen.

Assistant/Threads/WebApp.hs

@@ -6,6 +6,7 @@
  -}
 
 {-# LANGUAGE TemplateHaskell, MultiParamTypeClasses #-}
+{-# ViewPatterns, OverloadedStrings #-}
 {-# LANGUAGE CPP #-}
 {-# OPTIONS_GHC -fno-warn-orphans #-}