Properly format search result counts

Summary

Follow-up from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14689/diffs#note_201138388 / https://gitlab.com/gitlab-org/gitlab-ee/issues/12547

For searches using the DB the queries are limited to 1001 results and displayed as 1000+.

For searches using Gitaly (blobs/commits) and Elasticsearch (anything except users) we don't limit the queries but display the raw number, which doesn't look nice and takes up unnecessary space:

Proposal

Format the numbers using e.g. K/M/B units for thousands/millions/billions:

Considerations

• Should we use decimal numbers for millions/billions?
• Do we need to localize the units?

Involved components

• Gitlab::SearchResults#formatted_count
• Gitlab::Elastic::SearchResults#formatted_count