Resolve "Create release notes from closed milestone"
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Add a button "New Release Tag" to milestones/show if the milestone status is closed and no release tag has been created.

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Add a link to milestones/show description with the release tag (if exists)

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The tag's description includes the open and closed issues for that milestone.

Left todo:
- Add integration specs.
- When I click to create a new tag but give up and click cancel, it should return to
milestone#showinstead oftags#index. - Replace the inline fields with a Tag model. This wasn't done because would demand a big refactor on other parts of the system, for example in
CreateTagService, and I'd never make a big change like this without creating specs (which the time constraints wouldn't allow me to). - Even better release notes description formatting.
- The extras (Like grouping by labels).
Closes #24260 (moved)



