Use timezone ID instead of full VTIMEZONE in DB

[rfc2822]

Currently we use the full VTIMEZONE from the server in the Collection table.

However we only use the ID and have to extract it every time (and Calendar Provider doesn't support custom VTIMEZONEs anyway), so it seems more useful to have only the tz id there.

Requires DB migration.

(Not urgent, for somewhen)

Depends on #1044

• [x] Send calendar-timezone-id with MKCALENDAR
• [ ] Request and process calendar-timezone-id, if available (fall back to calendar-timezone)
• [ ] Store timezone ID instead of VTIMEZONE in database (+ migration)