#2192 Registration process (possibly) not marking status correctly / immediately 

Hi @aronasorman this is an initial fix for #2192.

Fixed:
1. Register button which still shows after user has registered.

I removed the `class=registered-only` attribute of the **Sync Now!** button.

I used the context variable `device.is_registered` to know if the device is already registered. 

Todos:

1. Replicate the registration process that is not marking status correctly.
If we're going with this approach here, then we can also remove `not-registered-only`.