EZP-23458: disable autosave preview when Preview link is hidden.

JIRA: https://jira.ez.no/browse/EZP-23458

When `HidePreviewLink` setting is `enabled` in ezautosave , there is no reason to fetch preview content and store it on the iframe.
This modifies the ajax request used for autosave to `savedraft` in this case, which does not return any content.