bitwarden_rs: Configure BURP client

This commit configures *bw0.pyrocufflink.blue* as a BURP client, so that
the Bitwarden data can be backed up.  A pre-backup script is used to
take a consistent snapshot of the SQLite database before copying it to
the BURP server.

group_vars/bitwarden_rs.yml

@@ -1 +1,4 @@
 bitwarden_domain: https://bitwarden.pyrocufflink.blue
+burp_backup_script_pre: backup-pre.sh
+burp_backup:
+- include: /var/lib/docker/volumes/bw-data/_data