Basic database setup & migrations

• Set up H2 database
• Create basic schema
• DAO code
• Skeleton for schema migrations

changed milestone to %Mailbox

added Sponsor 6 label

changed the description

added S: in progress label

mentioned in merge request !16 (merged)

closed with commit 37b77b35

changed milestone to %Mailbox: Database

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

mentioned in commit 37b77b35

added Mailbox label

marked this issue as related to #63 (closed)

added Weight: 5 label

added Mailbox app label