Enable X-FRAME-Option in HTTP response headers in order to provide clickjacking protection

Crowd is vulnerable to Clickjacking. That is, it is possible to frame crowd from a page hosted in a different domain and trick the user into performing an action they did not intend to perform, for example changing their display name.

This issue can be addressed by using the X-Frame-Options header and or through the CSP frame-ancestors directive. When fixing this issue we need to ensure that resources that need to be able to be framed are still allowed to be framed, e.g. gadget resources.