Add room favourite drag and drop to Vue left-menu

Add room favourite drag and drop to Vue left-menu

• Uses https://github.com/SortableJS/Vue.Draggable for drag and drop (seems to be the most popular and was easy to use)

On favourite persisting request failure, we show a notification and rollback:

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

Try the following with and without the feature toggle:

1. Drag rooms into the favourites list
2. Drag rooms around in the favourites list
3. Drag rooms out of the favourites list

Previous old left-menu code

Frontend

• https://gitlab.com/gitlab-org/gitter/webapp/blob/f623bb85a49b9b5458ae2cb914c22840b476b954/public/js/components/menu/room/dnd-controller.js
• https://gitlab.com/gitlab-org/gitter/webapp/blob/f623bb85a49b9b5458ae2cb914c22840b476b954/public/js/views/menu/room/favourite-collection/favourite-collection-view.js#L50-75
• https://gitlab.com/gitlab-org/gitter/realtime-client/blob/9c11abd8a1206b712272a99aed49c95f1cb10471/lib/room-collection.js#L25-76

Styles:

• https://gitlab.com/gitlab-org/gitter/webapp/blob/f623bb85a49b9b5458ae2cb914c22840b476b954/public/js/views/menu/room/favourite-collection/favourite-collection-view.less

Server:

• https://gitlab.com/gitlab-org/gitter/webapp/blob/f623bb85a49b9b5458ae2cb914c22840b476b954/modules/rooms/lib/recent-room-core.js#L22-92

Dev notes

• https://github.com/SortableJS/Vue.Draggable/blob/cf0aba17372e392fc569c1b9a90495f124e4cc70/tests/unit/vuedraggable.integrated.spec.js
• https://github.com/SortableJS/Vue.Draggable/blob/cf0aba17372e392fc569c1b9a90495f124e4cc70/tests/unit/vuedraggable.spec.js

---

Closes https://gitlab.com/gitlab-org/gitter/webapp/issues/2154