Better priority sorting

What does this MR do?

1. Change the priority sort to be milestone due date soonest + label priority.
2. Add a 'started' filter for milestones, which returns results from all milestones with a start date less than or equal today.

Are there points in the code the reviewer needs to double check?

All of it?

Why was this MR needed?

We had to create Next Patch Release to work around this limitation.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/28754.