* refactor the jmap package to split it into several files as the
jmap.api.go file was becoming too unwieldy
* refactor the Groupware handler function response to be a Response
object, to be more future-proof and avoid adding more and more
return parameters while handling "no content" response as well
* more godoc for the JMAP model
* add Email creation, updating, deleting (Email/set,
EmailSubmission/set)
* add endpoints
- POST /accounts/{accountid}/messages
- PATCH|PUT /accounts/{accountid}/messages/{messageid}
- DELETE /accounts/{accountid}/messages/{messageid}
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