fix some mixed space+tab indentation

This fixes all instances of " \t" in the code base. Most common case
seems to be after a "where" line; probably vim copied the two space layout
of that line.

Done as a background task while listening to episode 2 of the Type Theory
podcast.

Command/Migrate.hs

@@ -65,7 +65,7 @@ upgradableKey backend key = isNothing (Types.Key.keySize key) || backendupgradab
 perform :: FilePath -> Key -> Backend -> Backend -> CommandPerform
 perform file oldkey oldbackend newbackend = go =<< genkey
   where
-  	go Nothing = stop
+	go Nothing = stop
 	go (Just (newkey, knowngoodcontent))
 		| knowngoodcontent = finish newkey
 		| otherwise = stopUnless checkcontent $ finish newkey