Mention in non-joined room should show up in your left-menu (Vue)

Mention in non-joined room should show up in your left-menu (Vue)

If you haven't joined a room but someone mentions you, it should show up in your room list

Testing strategy

1. Add the feature toggle, node ./scripts/utils/feature-toggle.js --name vue-left-menu --description "Left menu using Vue (alpha)"
2. Enable the feature toggle
   • Enable: http://localhost:5000/api_web/features/vue-left-menu/1
   • Disable: http://localhost:5000/api_web/features/vue-left-menu/0
3. Have user1 with Gitter open
4. With user2, in a room that user1 hasn't joined, send a message with a mention of them @user1
5. With user1, notice how the room appears on the left-menu
6. Clicking on the room should open it and show a join room button