Feature request: status-page templates and maintenance statuses for events

[minkimipt]

Use case

Driving a planned maintenance's public status-page announcement through its lifecycle from the CLI: announced → in progress → completed.

What's missing

status-pages events create/update/resolve (added for #52) work for incidents, but for a maintenance:

• There's no way to pick a status-page template. The UI's Publish Incident form selects a template that renders the announcement from the incident's fields; the CLI requires a hand-written --message.
• The valid --status values for a maintenance event aren't documented, and resolve hardcodes status := "resolved" (events_mutations.go). It's unclear whether that maps to the maintenance Completed state the UI sets.

Why it matters

With a template the announcement stays consistent with what the UI produces (and with the custom fields on the incident); a hand-written message drifts. And if resolve doesn't produce Completed for a maintenance, the last step still needs the UI.

Proposed

rootly status-pages events create <incident-id> --status-page=<id> --template=<template-id> --notify-subscribers
rootly status-pages templates list --status-page=<id>
rootly status-pages events update <event-id> --status=in_progress
rootly status-pages events resolve <event-id>   # maps to "completed" for scheduled incidents

plus the accepted status values per incident kind in --help.