upgrade alerts

The webapp will check twice a day, when the network is connected, to see if
it can download a distributon upgrade file. If a newer version is found,
display an upgrade alert.

This will need the autobuilders to set UPGRADE_LOCATION to the url
it can be downloaded from when building git-annex. Only builds with that
set need automatic upgrade alerts.

Currently, the upgrade page just requests the user manually download
and upgrade it. But, all the info is provided to do automated upgrades
in the future.

Note that urls used will need to all be https.

This commit was sponsored by Dirk Kraft.

Assistant/Alert.hs

@@ -215,6 +215,26 @@ notFsckedAlert mr button = Alert
 	, alertData = []
 	}
 
+canUpgradeAlert :: AlertPriority -> AlertButton -> Alert
+canUpgradeAlert priority button = Alert
+	{ alertHeader = Just $ fromString $ concat
+		[ if priority >= High
+			then "An important upgrade of git-annex is available!"
+			else "An upgrade of git-annex is available."
+		]
+	, alertIcon = Just UpgradeIcon
+	, alertPriority = priority
+	, alertButton = Just button
+	, alertClosable = True
+	, alertClass = Message
+	, alertMessageRender = renderData
+	, alertCounter = 0
+	, alertBlockDisplay = True
+	, alertName = Just UpgradeAlert
+	, alertCombiner = Just $ dataCombiner $ \_old new -> new
+	, alertData = []
+	}
+
 brokenRepositoryAlert :: AlertButton -> Alert
 brokenRepositoryAlert = errorAlert "Serious problems have been detected with your repository. This needs your immediate attention!"