18213 Milestone path

What does this MR do?

Add human readable path to milestone.

With features:

• accepting params with/without #path param (mostly for API compatibility)
• supporting backward compatibility for old milestone#iid path (with 302 redirect)
• supporting dots in the path
• done by analogy with group#path

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

everything

Why was this MR needed?

there was the issue on it

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• 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 you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #18213 (moved)