Send and receive backup over network using QR code

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.

src/qr.rs

@@ -64,6 +64,9 @@ pub enum Qr {
     Account {
         domain: String,
     },
+    Backup {
+        ticket: sendme::provider::Ticket,
+    },
     WebrtcInstance {
         domain: String,
         instance_pattern: String,