Tag editing

Feature request [via Zendesk](https://hypothesis.zendesk.com/agent/tickets/2942)

> I am not trying to delete and rename tags on an individual annotation; I am trying to give a tag a different name. I would also prefer to delete a tag that I am not using for any annotations.

So, for example, if I've tagged a bunch of annotations with `foo` and realize I'd actually like those annotations to be tagged with `foobar`, I'd like to change the tag itself rather than manually updating each annotation.

And if I've been using the tag `fizz` and it's not useful to me, I'd like to remove it from all of my annotations at the same time.

No files, tests, or entry points are named in the issue. Start by locating the existing tag-management flow, then define completion as renaming a tag across all of the user's annotations and removing an unused tag from them in one operation.

Written by the indexing model from the issue text.