This adds functionality to send and receive a backup over the network
using a QR code.
The sender or provider prepares the backup, sets up a server that
waits for clients. It provides a ticket in the form of a QR code
which contains connection and authentication information.
The receiver uses the QR code to connect to the provider and fetches
backup, restoring it locally.
This makes the contact ID its own newtype instead of being a plain
u32. The change purposefully does not yet try and reap any benefits
from this yet, instead aiming for a boring change that's easy to
review. Only exception is the ToSql/FromSql as not doing that yet
would also have created churn in the database code and it is easier to
go straight for the right solution here.
