Avoid loading all labels when autocompleting quick action commands

What does this MR do and why?

Use an SQL exists query to check if there are available labels instead of loading all labels into memory.

This is slow for projects / groups with a lot of labels.

References

Related to #595401 (closed)

How to set up and validate locally

1. Type / in the comment text area of a work item or MR
2. Check the performance bar queries for the /<group>/<project>/-/autocomplete_sources/commands request

Before	After

The query should now be an existence query in the format: SELECT 1 as one ... LIMIT 1

MR acceptance checklist

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