Cheap workaround to force protocol in settings redirecturl

Quick and dirty workaround to #9.

To test: 
1. go to Admin > Services > Captive Portal 
2. Click the Redirect to Homepage checkbox and enter a valid hostname without http or https as in #9 bug report. On submit, the homepage field should now show http://<yourHostname>.
3. Click submit again. Homepage field should keep http://<yourHostname>.
4. Change homepage to https://<yourHostname> and click submit. Homepage should keep https://<yourHostname>.