* add endpoints for Mailboxes:
- PATCH mailboxes/{id}
- DELETE mailboxes/{id}
- POST mailboxes
* refactor the pkg/jmap and groupware framework to systematically
return a jmap.State out-of-band of the per-method payloads, since
they are almost always present in JMAP responses, which lead to the
artificial creation of a lot of composed struct types just to also
return the State; on the downside, it adds yet another return
parameter
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