Add a Referrer-Policy header setting to the security admin settings

Regression testing: Visit Site Administration > Security > HTTP Security (/admin/settings.php?section=httpsecurity). Ensure the 'Referrer Policy' is set to 'Browser Default'. In another tab, open the dev tools (F12), then navigate to the 'Network' tab. Click the Document filter, to hide all the JS stuff happening. In the new tab, visit the dashboard (/my/) Now click on the new entry in the dev tools, then click the headers tab. Verify in the general section, the Referrer policy is 'no-referrer-when-downgrade' Verify that in the 'Response headers' section, there is no 'Referrer-Policy' header. Header testing: In the tab with the HTTP settings still open, set the 'Referrer Policy' setting to 'no-referrer' and save changes. In the other tab with dev tools open, reload the page. Click on the new entry in the dev tools Verify the response headers now contains the header 'Referrer-Policy: no-referrer' Repeat these steps for all remaining select options, and verify that the header content equals the select option selected.