Enabled Secure Cookies, cookies sent only when HTTPS is present

I realised that the csesoc website didn't have the "secure" flag on the session token it was issuing. I added it in the django settings, however have not tried or checked if my changes are actually working.

*Before accepting this pull request, I'd suggest that someone tests that the setting that I just added, does indeed add the "secure" flag to the session cookie when logged in, and also does not break site functionality*

Thanks!
In future, I will be testing if my changes are functional, but for the time being, I thought I'd just push this out.