Fetch issues / MRs / epics count in the sidebar with GraphQL

What does this MR do and why?

This MR is the frontend part of the fix for the bug where visiting a group with many subgroups/issues/epics (less than 1k as when it's more we cache) may cause a timeout issue when trying to render the Issues/MR count.

When the flag asyncSidebarCounts is enabled, we will fetch the issues / MRs / epics count in the sidebar with GraphQL.

References

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MR acceptance checklist

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Screenshots or screen recordings

No UI changes. UI should work the same and have no regressions.

asyncSidebarCount

How to set up and validate locally

With the flag async_sidebar_counts disabled or enabled, expect no regressions or UI changes. The only change is where we fetch data from.

1. Enable the flag async_sidebar_counts
2. Check any group and `project routes. For example:

• http://localhost:3000/:group
• http://localhost:3000/groups/:group/-/issues and mrs, epics
• http://localhost:3000/:group/:project
• http://localhost:3000/:group/:project/-/issues and mrs

3. expect that getAsyncSuperSidebarData graphQL is queried and the count for these items is rendered properly
4. expect that for thousands and higher numbers, we continue to shorten them (eg. 81523 displays 81.5k)

Related to #501679 (closed)