dev.knotless.labelItem

Rewrites the dev.knotless.label record about a pull or an issue in the account logged in to this appview's web pages, as `knotless pull label` and `knotless pull label --undo` do: the repository's owner and collaborators put on any label it defines, and its triagers those of their scopes. Putting on an exclusive label takes the others of its scope off. Requires the session cookie the pages' login sets.