Commune Subscriber tags API
A tag segments a newsletter's audience. Sending an article to a tag stamps that article with an audience, which is what makes it invisible to everyone outside it. Named for the subscribers it is applied to, because a tag called `Tags` inside a document made of tags says nothing. Applying and removing a tag are writes, and they are grants and revocations of access to whatever articles that segment was addressed to, not only labels. Creating, renaming and retiring a tag are not operations here yet.