Add room search to Vue left-menu

Add room search to Vue left-menu

Fix https://gitlab.com/gitlab-org/gitter/webapp/issues/2155

Dev notes:

• https://gitlab.com/gitlab-org/gitter/webapp/blob/e7b6cf23e4f7b2783c8b21ef19cf0f5e9967d415/public/js/collections/left-menu-search-rooms-and-people.js

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

With the feature-toggle on

1. Open the search tab
2. Search for a room you have joined and notice it listed. You will see errors for fetching rooms/people because Elasticsearch doesn't work locally in dev
3. You can test on beta-staging for successful searches
   • Look out if rooms are duplicated in the results

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1. Search for something in one room
2. Switch rooms with the left menu
3. Then switch back to the search and notice it re-searched messages in that room

Closes #2155 (closed)