Implement PLT-6246 - Confirm before sending (at)all, (at)channel message (#6250)

* implement PLT-6246 - Confirm before sending (at)all, (at)channel message

* refactor per review

* add constant to define the notify all modal

webapp/utils/constants.jsx

@@ -337,6 +337,7 @@ export const Constants = {
     },
 
     SPECIAL_MENTIONS: ['all', 'channel', 'here'],
+    NOTIFY_ALL_MEMBERS: 5,
     CHARACTER_LIMIT: 4000,
     IMAGE_TYPES: ['jpg', 'gif', 'bmp', 'png', 'jpeg'],
     AUDIO_TYPES: ['mp3', 'wav', 'wma', 'm4a', 'flac', 'aac', 'ogg'],