tickets: set and show labels and assignees #1354

Tickets carry tangledLabels and assignees (org.tangled.track.ticket), and the web app sets both. atgc keeps them when it edits a ticket, but can't set them.

• Add --label and --assignee to pr create, issue create, pr edit and issue edit, with a way to remove each.
• Show both in pr view and issue view, and in their --json.
• Filter pr list and issue list by them.

Mirror the web app's setTicketLabels and its assignee writes in Tangled's web/src/lib/api.