Fix milestone references with HTML entities in the name

What does this MR do?

When a milestone name contained an HTML entity that would be escaped (&, <, >), then it wasn't possible to refer to this milestone by name, or use it in a quick action.

This already worked for labels, but not for milestones. We take care to re-escape un-matched milestones, too.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Style guides

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/62114.