WL-3230 Persist existing file settings

Currently the webdav server will delete the old file and create a new one, this will result in lost settings.

For example if a file has Oxford or Public access then an edit through webdav will overwrite this.

This code is not ideal because `editResource` in `ContentHostingService` will not through an ID unused exception if the file does not exist. Using the `getResource` method to detect new files instead.