webapp: New --listen= option allows running the webapp on one computer and connecting to it from another.

Does not yet use HTTPS. I'd need to generate a certificate, and I'm not
sure what's the best way to do that.

Assistant/WebApp/Types.hs

@@ -26,6 +26,7 @@ import Yesod
 import Yesod.Static
 import Text.Hamlet
 import Data.Text (Text, pack, unpack)
+import Network.Socket (HostName)
 
 publicFiles "static"
 
@@ -38,6 +39,7 @@ data WebApp = WebApp
 	, getStatic :: Static
 	, postFirstRun :: Maybe (IO String)
 	, noAnnex :: Bool
+	, listenHost ::Maybe HostName
 	}
 
 instance Yesod WebApp where