Refactor roles into Roles.js

So that the mapping between a numerical power level and a "role" are done in one place. PowerSelector.js has been modified to use the same mapping.

src/Roles.js

@@ -0,0 +1,29 @@
+/*
+Copyright 2017 Vector Creations 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.
+*/
+export const LEVEL_ROLE_MAP = {
+    undefined: 'Default',
+    0: 'User',
+    50: 'Moderator',
+    100: 'Admin',
+};
+
+export function textualPowerLevel(level, userDefault) {
+    if (LEVEL_ROLE_MAP[level]) {
+        return LEVEL_ROLE_MAP[level] + (level !== undefined ? ` (${level})` : ` (${userDefault})`);
+    } else {
+        return level;
+    }
+}