API endpoint for adding a contact

The endpoint should return the contact's auth token and the incoming and outgoing folder IDs.

Depends on #43 (closed), #44 (closed), #45 (closed). Subtask of #4 (closed).

changed milestone to %Mailbox: Contact management API

added Sponsor 6 T: task labels

added Mailbox label

mentioned in issue briar#2183 (closed)

marked this issue as related to briar#2183 (closed)

added S: in progress label

mentioned in merge request !24 (merged)

marked this issue as related to #4 (closed)

assigned to @sebkur

mentioned in commit 91bd3c2a

closed with merge request !24 (merged)

added R: fixed label and removed S: in progress label

added Weight: 3 label

added Mailbox app label