Implement Faceted Search

Currenlty, filtering of search results (Issues, Merge Requests) is pretty limited. For example we can't filter by multiple labels, milestones etc. Would be great if search filtering used facets similar to how the modern websites do it these days (Amazon.com, eBay.com etc.). Users would check and uncheck any facets that they want to filter results by. I am including a screenshot to show the high-level idea.