* move ContactCard from jscontact to jmap, as it is actually a JMAP
specification item, but also was causing too many issues with
circular references from jscontact -> jmap
* introduce Foo, Idable, GetRequest, GetResponse, etc... types and
generics
* first attempt at a Foo factory type for Mailboxes, needs to be
expanded to further minimize repetition
* add more specialized template functions to avoid repetition
* introduce ChangesTemplate[T] for *Changes structs
* add Groupware APIs for creating and deleting addressbooks
* add Groupware APIs for creating and deleting calendars
* add JMAP APIs for creating and deleting addressbooks, calendars
* add JMAP APIs to retrieve Principals
* fix API tagging
* move addressbook JMAP APIs into its own file
* move addressbook Groupware APIs into its own file
* implement ContactCard retrieval endpoint for syncing
* re-implement that endpoint for Email too
* fix the Mailbox changes endpoint to actually return changes about
Mailboxes, and not about Emails
* when querying the diff of Mailboxes without any prior state, return
an error since the result is not what one would expect
* introduce the 'changes' API tag and group
* refactor the successful response functions to consistently return an
object type and object state whenever possible
* move the syncing endpoints under /accounts/*/changes/ for better
clarity, e.g. /changes/emails instead of /emails/mailbox/*/changes
