Remove the need for a directory for db export

Plus on import use the context directory.  We can actually write there
just fine.

src/qr.rs

@@ -66,7 +66,17 @@ pub enum Qr {
     Account {
         domain: String,
     },
+    /// Provides a backup that can be retrieve.
+    ///
+    /// This contains all the data needed to connect to a device and download a backup from
+    /// it to configure the receiving device with the same account.
     Backup {
+        /// Printable version of the provider information.
+        ///
+        /// This is the printable version of a `sendme` ticket, which contains all the
+        /// information to connect to and authenticate a backup provider.
+        ///
+        /// The format is somewhat opaque, but `sendme` can deserialise this.
         ticket: sendme::provider::Ticket,
     },
     WebrtcInstance {