global webapp redirects, to finish upgrades

When an automatic upgrade completes, or when the user clicks on the upgrade
button in one webapp, but also has it open in another browser window/tab,
we have a problem: The current web server is going to stop running in
minutes, but there is no way to send a redirect to the web browser to the
new url.

To solve this, used long polling, so the webapp is always listening for
urls it should redirect to. This allows globally redirecting every open
webapp. Works great! Tested with 2 web browsers with 2 tabs each.
May be useful for other purposes later too, dunno.

The overhead is 2 http requests per page load in the webapp. Due to yesod's
speed, this does not seem to noticibly delay it. Only 1 of the requests
could possibly block the page load, the other is async.

Assistant/Threads/UpgradeWatcher.hs

@@ -25,8 +25,6 @@ import Assistant.DaemonStatus
 import Assistant.WebApp.Types
 import qualified Build.SysConfig
 #endif
-import qualified Annex
-import Types.Distribution
 
 import Control.Concurrent.MVar
 import Data.Tuple.Utils
@@ -102,7 +100,7 @@ handleUpgrade urlrenderer = do
 	-- (For example, other associated files may be being put into
 	-- place.)
 	liftIO $ threadDelaySeconds (Seconds 120)
-	ifM (liftAnnex $ (==) AutoUpgrade . annexAutoUpgrade <$> Annex.getGitConfig)
+	ifM autoUpgradeEnabled
 		( do
 			debug ["starting automatic upgrade"]
 			unattendedUpgrade
@@ -118,9 +116,12 @@ handleUpgrade urlrenderer = do
 showSuccessfulUpgrade :: UrlRenderer -> Assistant ()
 showSuccessfulUpgrade urlrenderer = do
 #ifdef WITH_WEBAPP
-	button <- mkAlertButton True
-		(T.pack "Enable Automatic Upgrades")
-		urlrenderer ConfigEnableAutomaticUpgradeR
+	button <- ifM autoUpgradeEnabled 
+		( pure Nothing
+		, Just <$> mkAlertButton True
+			(T.pack "Enable Automatic Upgrades")
+			urlrenderer ConfigEnableAutomaticUpgradeR
+		)
 	void $ addAlert $ upgradeFinishedAlert button Build.SysConfig.packageversion
 #else
 	noop