Refactor aggregations calls for issues

What does this MR do and why?

Related to #461874 (closed)

Behind feature flag issues_search_refactor and broken up in a few MRs to reduce review complexity and risk:

Description	MR Link
Refactor Empty search results class	!157210 (merged)
Rubocop fixes for search_results_spec.rb	!157517 (merged)
Refactor search calls for issues	!155516 (merged)
Refactor aggregations calls for issues	⭐ !157523 (merged)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

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How to set up and validate locally

test with and without the show_issue_refactor feature flag

1. enable elasticsearch for gdk and index everything using gitlab:elastic:index rake task
2. apply labels to a few issues
3. perform issue search
4. verify that labels load in the drop down, and that they match the labels from the results
5. select one of the labels and apply
6. ensure the labels filter is applied to the search results