Cache MRs count on milestone list page as well as simplify queries

What does this MR do?

This change caches the result of SQL queries as well as makes the queries simpler by making the count independent of the current user. It's a logical step in the light of 99eec020

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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Security

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Related to #325598 (closed)