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/WebApp/Configurators/Upgrade.hs

@@ -12,7 +12,6 @@ module Assistant.WebApp.Configurators.Upgrade where
 import Assistant.WebApp.Common
 import qualified Annex
 import Types.Distribution
-import Assistant.WebApp.OtherRepos
 import Assistant.Upgrade
 import Utility.HumanTime
 import Git
@@ -28,20 +27,14 @@ getConfigStartUpgradeR d = page "Upgrade git-annex" (Just Configuration) $ do
 	$(widgetFile "configurators/upgrade/start")
 
 {- Finish upgrade by starting the new assistant in the same repository this
- - one is running in, and redirecting to it.
- -
- - Note that only the browser tab that requested this page gets redirected.
- - If the user has multiple web browser tabs open to the webapp,
- - the others will show the upgradingAlert, and keep running until
- - this process is terminated.
- -}
+ - one is running in, and redirecting to it. -}
 getConfigFinishUpgradeR :: Handler Html
 getConfigFinishUpgradeR = do
 	liftAssistant prepUpgrade
-	liftIO postUpgrade `after` startnewprocess
-  where
-	startnewprocess = switchToAssistant
+	url <- liftIO . newAssistantUrl
 		=<< liftAnnex (repoLocation <$> Annex.gitRepo)
+	liftAssistant $ postUpgrade url
+	redirect url
 
 getConfigEnableAutomaticUpgradeR :: Handler Html
 getConfigEnableAutomaticUpgradeR = do