Only enable HSTS header for HTTPS and port 443

### What does this MR do?

This MR adds a check that the port used is 443, in addition to HTTPS being enabled, when activating the HSTS header.

### Why was this MR needed?

If a user is using a non-standard port for SSL, enabling this header would send clients to port 443 when that port is invalid.

### What are the relevant issue numbers?

Closes https://github.com/gitlabhq/gitlabhq/issues/9449

See merge request !966