Improve various parts of Dashboard::ProjectsController

What does this MR do?

This MR adds various performance improvements for the controller Dashboard::ProjectsController, reducing the time spent in queries and the number of queries being executed. For example, in my development environment the projects dashboard page went from 30-something queries down to just 12 queries.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/27376 for more information.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides