gui: Add button for changing backup group owner

Extension of fix #2847

Adds an action button to the datastore content view,
to change the owner of a backup.

Signed-off-by: Dylan Whyte <d.whyte@proxmox.com>

www/Makefile

@@ -52,6 +52,7 @@ JSSRC=							\
 	datastore/Prune.js				\
 	datastore/Content.js				\
 	datastore/Panel.js				\
+	BackupGroupChangeOwner.js			\
 	ServerStatus.js					\
 	ServerAdministration.js				\
 	Dashboard.js					\