Fix Content-Security-Policy for Element

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1154

According to
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy,
having both a header and the `<meta>`-tag provided by Element itself is
not a problem. The 2 CSP policies get combined.

roles/matrix-nginx-proxy/templates/nginx/conf.d/matrix-client-element.conf.j2

@@ -12,7 +12,7 @@
 	add_header X-Content-Type-Options nosniff;
 	add_header X-XSS-Protection "{{ matrix_nginx_proxy_xss_protection }}";
 	add_header X-Frame-Options SAMEORIGIN;
-	add_header Content-Security-Policy "frame-ancestors 'none'";
+	add_header Content-Security-Policy "frame-ancestors 'self'";
 
 	{% if matrix_nginx_proxy_floc_optout_enabled %}
 		add_header Permissions-Policy interest-cohort=() always;