Convert PowerSelector to use mxField instead

res/css/views/elements/_PowerSelector.scss

@@ -0,0 +1,25 @@
+/*
+Copyright 2019 New Vector 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_PowerSelector {
+    width: 100%;
+}
+
+.mx_PowerSelector .mx_Field select,
+.mx_PowerSelector .mx_Field input {
+    width: 100%;
+    box-sizing: border-box;
+}