Add API docs to ContactsManager

Copy over relevant API docs from the API spec into the ContactsManager implementation

changed milestone to %Mailbox: Contact management API

added Sponsor 6 label

assigned to @sebkur

mentioned in merge request !24 (merged)

added Mailbox label

added Weight: 1 label

changed due date to December 13, 2021

mentioned in merge request !49 (merged)

added S: in code review label

mentioned in commit bf3a3c44

closed with merge request !49 (merged)

added Mailbox app label

added R: fixed label and removed S: in code review label