Updates to nginx config to ensure all requests get an HTTPS response.

Relating to ExamTime/#6307.

Previously the proxy pass config handled HTTP and HTTPS traffic
differently. This caused an issue when we added an ELB and used this to
terminate our SSL. The ELB took an HTTPS request and passed on a
decrypted request to our application. As a consequence the application
server returned an HTTP response (i.e. with http: assets etc). This led
to mixed content warning for the blog pages.

However, as we 301 all HTTP traffic to HTTPS, then it is not really
necessary to handle HTTP/S differently. So this change changes our nginx
recipe to apply the same config to both custom.conf and custom.ssl.conf.