Fix Albumik frontend assets and YunoHost upgrade scripts

scripts/backup

@@ -1,6 +1,7 @@
 #!/bin/bash
 set -euo pipefail
-source ./_common.sh
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+source "$SCRIPT_DIR/_common.sh"
 
 mkdir -p "$YNH_BACKUP_DIR"
 tar -C / -czf "$YNH_BACKUP_DIR/albumik-data.tar.gz" "${data_dir#/}" "${config_dir#/}" "${install_dir#/}" 2>/dev/null || true