switch initial spinner to mx_Spinner

src/skins/vector/css/atoms/Spinner.css

@@ -0,0 +1,25 @@
+/*
+Copyright 2015 OpenMarket Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+.mx_Spinner {
+    display: -webkit-flex;
+    display: flex;
+    -webkit-align-items: center;
+    -webkit-justify-content: center;
+    align-items: center;
+    justify-content: center;
+    height: 100%;
+}