Fix some N+1 queries related to Admin Dashboard, User Dashboards and Activity Stream

What does this MR do?

Fix some occurrences of N+1 queries related to either Projects or Namespaces.

This improved few routes:

• /admin
• /
• /dashboard/projects
• /dashboard/activity
• /explore

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.

fixed gitlab-org/gitlab-ce#55106