Merge pull request #106 from mattermost/mm-1119

fixes mm-1119 adds @all and @channel mention capabilites

web/react/utils/constants.jsx

@@ -36,6 +36,7 @@ module.exports = {
     SERVER_ACTION: null,
     VIEW_ACTION: null
   }),
+  SPECIAL_MENTIONS: ['all', 'channel'],
   CHARACTER_LIMIT: 4000,
   IMAGE_TYPES: ['jpg', 'gif', 'bmp', 'png'],
   AUDIO_TYPES: ['mp3', 'wav', 'wma', 'm4a', 'flac', 'aac'],