* 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
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