Add option to disable caching for HTTP responses

Some users are requesting the Cache-Control: no-store HTTP response header to be enabled for security reasons [1]. As NiFi is a SPA, this could significantly impact performance and we suspect it has minimal security impact. Adding this response header would necessitate an admin-configurable setting to enable/disable the header with the default value false (continuing existing behavior).

[1] https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control