Read hostkey from GET or POST

This commit applies the fix from
https://github.com/tsudoko/anki-sync-server/pull/60/files

However it using a shorter version by utilizing the params attribute of
the webob request. The params attribute combines the get and post params

src/ankisyncd/sync_app.py

@@ -499,7 +499,7 @@ class SyncApp:
     def __call__(self, req):
         # Get and verify the session
         try:
-            hkey = req.POST['k']
+            hkey = req.params['k']
         except KeyError:
             hkey = None