* in the JMAP API as well as in several places in the Groupware
framework, use a single jmap.Response[T] object to return the
payload, the language, the session state and the etag/state instead
of individual multi-valued return values
* adds creating addressbooks, calendars, mailboxes
* adds deleting mailbox, event, identity
* adds modifying an email
* introduce template functions for the Groupware API in templates.go,
and use those in route function implementations whenever possible
* add capability checking for mail, quota, blobs
* adds Changes interface
* adds JmapResponse interface
