Follow-up: NOT queries for labels are too slow

In !27639 (comment 334758634) it was noted that #by_negated_label in IssuableFinder is a bit too slow, and we should look into it and improve it.

Proposed next steps:

1. Compile a report of all actual product queries originating from the filter bar across all instances if possible. Study how people are filtering and what they are trying to accomplish to see if there are any quick wins.
2. Revisit the underlying architecture (and probably UX) of search/filtering so it is more performant. This might require switching to something like Elastic but it's ultimately up to Engineering to decide the optimum path forward.