Create a tag from a milestone

What does this MR do?

Introduce a way to create a tag/release from a milestone page, with some fields pre-filled. The tag name is prefilled from the milestone's title and the release notes field is simply a list of the milestone's issues, grouped by their state (open or closed).

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

1. Building the tag_name from milestone's title is a simple "parametrization". I went this way because it seems error prone to try to follow the ref format spec, it can change in unpredictable ways. I'm open to discussion.
2. There are no specs yet covering changes in the milestone show page.
3. Release notes field is not internationalized

In a high level, I think there is room for improvement on the BuildService. I am following the pattern of one service per controller action (visible in other parts of GiLab), but I'm not quite happy with that.

Why was this MR needed?

This address a feature request #24260 (moved).

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • 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
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Issue: #24260 (moved)