refactor custom actions

SFTPGo is now fully auditable, all fs and provider events that change
something are notified and can be collected using hooks/plugins.

There are some backward incompatible changes for command hooks

sftpgo.json

@@ -160,6 +160,7 @@
     "users_base_dir": "",
     "actions": {
       "execute_on": [],
+      "execute_for": [],
       "hook": ""
     },
     "external_auth_hook": "",