Internationalize the moved path redirect banner

What does this MR do and why?

The redirect notice shown after visiting an old project, group, or user path was built with routable.class.to_s.titleize inside a plain heredoc, so it was never wrapped in a translation function and always rendered in English.

This replaces it with one complete translatable sentence per routable type (Project, Group, User, plus a generic fallback), using named %{old_path} / %{new_path} placeholders, and adds the new strings to locale/gitlab.pot. The English output is unchanged.

References

Closes #612312 (closed)

How to set up and validate locally

1. Rename or transfer a project so a redirect route exists for its old path.
2. Visit the old path. You are redirected, and the notice reads the same as before in English.
3. Switch your preferred language: the notice is now picked up for translation.

Existing specs that assert the exact notice text (projects, labels, groups, milestones, users controllers) cover the unchanged English output.

MR acceptance checklist

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