Client Controls Fixes

Replaces #185.

Fixes #184, #189 

To test:
1. In your browser, go to Admin > Client Controls and change the Time Until Welcome Page is Shown to 1 hour.
2. "Seconds" should not be an available option.
3. Click Save, then Save & Apply.
4. SSH in and run `ndsctl status`. If you see actual nodogsplash data, the fix worked. If the fix didn't work you will see `ndsctl: nodogsplash probably not started (Error: Connection refused)`.
5. Open /etc/nodogsplash/nodogsplash.conf. ClientIdleTimeout and ClientForceTimeout should each be set to 60.
6. In your browser, set Time Until Welcome Page to 1 day. ClientIdleTimeout/ClientForceTimeout should be 1440.