Use header to expose healthcheck status instead of status code

This is mostly to prevent it sending me emails on every GET of the
page when erroring

cmsmanage/views.py

@@ -88,5 +88,5 @@ def healthcheck(request: HttpRequest):
     return HttpResponse(
         ("OK: " if all_ok else "CRITICAL: ") + "\n".join(messages),
         content_type="text/plain",
-        status=200 if all_ok else 500,
+        headers={"X-CMSManage-Status": "OK"},
     )