webapp: Add UI to stop assistant.

Would like to also have restart UI, but that's rather harder to do,
seems it'd need to start another copy of the webapp, and redirect the
browser to its new url, but running two assistants in the same repo at
the same time isn't good.

Assistant/WebApp/Control.hs

@@ -0,0 +1,28 @@
+{- git-annex assistant webapp control
+ -
+ - Copyright 2012 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU AGPL version 3 or higher.
+ -}
+
+{-# LANGUAGE CPP, TypeFamilies, QuasiQuotes, MultiParamTypeClasses, TemplateHaskell, OverloadedStrings, RankNTypes #-}
+
+module Assistant.WebApp.Control where
+
+import Assistant.WebApp.Common
+
+import Control.Concurrent
+import System.Posix (getProcessID, signalProcess, sigTERM)
+
+getShutdownR :: Handler RepHtml
+getShutdownR = page "Shutdown" Nothing $
+	$(widgetFile "control/shutdown")
+
+getShutdownConfirmedR :: Handler RepHtml
+getShutdownConfirmedR = page "Shutdown" Nothing $ do
+	{- Wait 2 seconds before shutting down, to give the web page time
+	 - to display. -}
+	void $ liftIO $ forkIO $ do
+		threadDelay 2000000
+		signalProcess sigTERM =<< getProcessID
+	$(widgetFile "control/shutdownconfirmed")