Add merge request data to the progress information on Milestones pages.

What does this MR do?

Adds data about merge requests to the Milestone show view. It previously only showed something like 3 issues: 2 open and 1 closed 20% complete, now it shows 3 issues: 2 open and 1 closed 2 merge requests: 2 open and 0 merged 20% complete.

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

I don't think so.

Why was this MR needed?

The milestone progress % didn't accurately reflect the open/merged MRs.

What are the relevant issue numbers?

Fixes #13980 (closed)

Screenshots (if relevant)

Before:

After:

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

cc: @jschatz1