* 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
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