dev.knotless.defineLabel

Adds the label to the repository's dev.knotless.repo record, in place of any of its name, as `knotless label define` does. Its owner can, logged in to this appview's web pages: the appview writes with the session it keeps for that account, which the pages' session cookie names. For an organization's repository, so can an admin who joined it, logged in to the pages or with a service token from their own PDS for this method: the appview's gateway, which the organization delegated its writes to, writes as the organization.