Improved log in/out handling

Nginx.conf:
- Remove HttpOnly attribute to be able to check if user has logged in
- Prevent log out from redirecting to non-HTTPS page
- Don't overwrite account-login-form hidden input element values
- Force request-header field Host to $proxy_host (Nginx bug?)
- Set Referer to $proxy_host for correct user authentication

HTMLTableHandler.js:
- Enable/disable place ad button when user logs in/out