Preload group ancestor to avoid N+1

What does this MR do?

For getting number of epics in a group and all its subgroups, we have to first check which subgroups are visible to the user. This permission check is not very effective because multiple additional SQL queries are done.

Because two of these extra queries are done because we need to get root ancestor for each group, we can optimize it be presetting root ancestor for all groups before doing permission check.

Related to #33638 (closed)

Related to #37368 (closed)

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Does this MR meet the acceptance criteria?

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