cli: unregistered key, issue create flags, mr show plurals, repo readme, mirror time !492

Smaller CLI findings from the UX review.

• An unregistered key is told its fingerprint and the real host, with both paths: add the key at <site URL>/settings#keys, or ssh git@<host> register ….
• issue create takes --label (repeatable), --milestone and --assignee (repeatable); they need write access, are resolved before the issue is created (a typo creates nothing), and go through the same code as issue label/milestone/assign, so the assignee is notified and the events are recorded. Help says $EDITOR opens when neither --body nor --file is given. issue assign now resolves every name before changing anything.
• mr show counts multi-row sections: commits (7):, checks (2):, reviews (3):.
• repo readme <owner/name> [--ref] prints a repository's README, chosen in the web page's order (control.PickReadme, shared with the web).
• repo show prints the mirror's last sync to the second.
• Users and Parity pages; CHANGELOG.

Stacked on !485 (cli-ux-help).

Closes #268