Store contacts in mailbox DB

Create a table in the mailbox DB for storing contacts, including:

• Integer contact ID provided by Briar
• Auth token
• Incoming and outgoing folder IDs

changed milestone to %Mailbox: Database

added Sponsor 6 T: task labels

assigned to @sebkur

mentioned in issue #57 (closed)

mentioned in issue #58 (closed)

added Mailbox label

mentioned in merge request !19 (closed)

added S: in progress label

mentioned in merge request !24 (merged)

marked this issue as related to #4 (closed)

mentioned in commit 91bd3c2a

closed with merge request !24 (merged)

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

added Weight: 1 label

added Mailbox app label