Eager load namespace owners for project dashboards

What does this MR do?

This fixes an N+1 query problem in Dashboard::ProjectsController where we'd query the owner of a namespace separately.

This fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/27376 (at least for the time being) since I don't see any other improvements we can make that are specific to this page.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together