EZP-23074: Legacy formToken controller, render csrf-token meta tag

JIRA: https://jira.ez.no/browse/EZP-23074

Legacy extensions using js/jquery/ezjscore expect a 'ezxform_token_js' tag, which is not displayed without a legacy pagelayout.
This PR adds a controller method in `EzPublishLegacyBundle` to render the csrf-token meta tag.