[Nginx] Reduce config duplication

It does not make sense having a seperate server block for both http
and https.
According to the nginx doc [1], using the same server block for both
should work.

[1] http://nginx.org/en/docs/http/configuring_https_servers.html#single_http_https_server

data/conf/nginx/templates/listen_ssl.template

@@ -1,2 +1,2 @@
-listen ${HTTPS_PORT} http2;
-listen [::]:${HTTPS_PORT} http2;
+listen ${HTTPS_PORT} ssl http2;
+listen [::]:${HTTPS_PORT} ssl http2;