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.

Build/Configure.hs

@@ -7,7 +7,7 @@ import Data.List
 import System.Process
 import Control.Applicative
 import System.FilePath
-import System.Environment
+import System.Environment (getArgs)
 import Data.Maybe
 import Control.Monad.IfElse
 import Data.Char
@@ -17,11 +17,13 @@ import Build.Version
 import Utility.SafeCommand
 import Utility.Monad
 import Utility.ExternalSHA
+import Utility.Env
 import qualified Git.Version
 
 tests :: [TestCase]
 tests =
 	[ TestCase "version" getVersion
+	, TestCase "UPGRADE_LOCATION" getUpgradeLocation
 	, TestCase "git" $ requireCmd "git" "git --version >/dev/null"
 	, TestCase "git version" getGitVersion
 	, testCp "cp_a" "-a"
@@ -90,6 +92,11 @@ testCp k option = TestCase cmd $ testCmd k cmdline
 	cmd = "cp " ++ option
 	cmdline = cmd ++ " " ++ testFile ++ " " ++ testFile ++ ".new"
 
+getUpgradeLocation :: Test
+getUpgradeLocation = do
+	e <- getEnv "UPGRADE_LOCATION"
+	return $ Config "upgradelocation" $ MaybeStringConfig e
+
 getGitVersion :: Test
 getGitVersion = Config "gitversion" . StringConfig . show
 	<$> Git.Version.installed