use openTempFile from file-io

And follow-on changes.

Note that relatedTemplate was changed to operate on a RawFilePath, and
so when it counts the length, it is now the number of bytes, not the
number of code points. This will just make it truncate shorter strings
in some cases, the truncation is still unicode aware.

When not building with the OsPath flag, toOsPath . fromRawFilePath and
fromRawFilePath . fromOsPath do extra conversions back and forth between
String and ByteString. That overhead could be avoided, but that's the
non-optimised build mode, so didn't bother.

Sponsored-by: unqueued

Utility/WebApp.hs

@@ -187,7 +187,9 @@ insertAuthToken extractAuthToken predicate webapp root pathbits params =
  - to avoid exposing the secret token when launching the web browser. -}
 writeHtmlShim :: String -> String -> FilePath -> IO ()
 writeHtmlShim title url file = 
-	viaTmp (writeFileProtected . toRawFilePath) file $ genHtmlShim title url
+	viaTmp (writeFileProtected . fromOsPath)
+		(toOsPath $ toRawFilePath file) 
+		(genHtmlShim title url)
 
 genHtmlShim :: String -> String -> String
 genHtmlShim title url = unlines