Rename a tag

Changes what a segment is called. Needs audience: write, and is a write.

name is the only thing about a tag that can be changed, and changing it moves nothing else. The people holding the tag still hold it, and every article addressed to it is still addressed to it, because an article names a tag by id rather than by name. A rename is a label, never a change of who may read what.

The new name has to be free among this newsletter's live tags; one a live tag already has answers 422.

A retired tag cannot be renamed, and answers 422. A retirement is the surviving record of what an already sent article was addressed to, and renaming it would relabel an audience that has already gone out.

Publishes no event.