Turn the chosen username to lowercase

So that the user doesn't get dumped to a CSS-less error page telling
them that the localpart doesn't match the regexp.

matrix_synapse_saml_mozilla/res/script.js

@@ -65,7 +65,9 @@ let submitUsername = function(username) {
     return;
   }
 
-    let check_uri = 'check?' + buildQueryString({"username": username});
+    // Since we're trying to register the username converted to lower case, check the
+    // availability with the right case.
+    let check_uri = 'check?' + buildQueryString({"username": username.toLowerCase()});
     fetch(check_uri, {
         "credentials": "include",
     }).then((response) => {

matrix_synapse_saml_mozilla/username_picker.py

@@ -155,6 +155,8 @@ class SubmitResource(AsyncResource):
             _return_html_error(400, "missing username", request)
             return
         localpart = request.args[b"username"][0].decode("utf-8", errors="replace")
+        # Convert the username to lower case.
+        localpart = localpart.lower()
         logger.info("Registering username %s", localpart)
         try:
             registered_user_id = await self._module_api.register_user(