Add code for revision cleanup (#6442)

* Add initial code for revision cleanup

* Some improvements - code cleanup

* Cleanup logging

* Add button in admin backend to cleanup revisions of a specific pad

* Disable cleanup by default and show errors in admin area

* Improve cleanup code

* Load revisions for cleanup in parallel

* Consider saved revisions during pad cleanup

settings.json.docker

@@ -171,6 +171,14 @@
    */
   "showSettingsInAdminPage": "${SHOW_SETTINGS_IN_ADMIN_PAGE:true}",
 
+  /*
+   * Settings for cleanup of pads
+   */
+  "cleanup": {
+    "enabled": false,
+    "keepRevisions": 5
+  },
+
   /*
     The authentication method used by the server.
     The default value is sso