Reactivate "Check App Update" button

[Re4son]

@PaulSec,
The "Check App Update" button is currently greyed out because of an issue with the update process.

A decision to install the nethunter-app as privileged app across the board is still outstanding #207 (comment).
Assuming we would agree on installing the app under /system/priv-app, could we re-enable that button and install the latest release from the github release page?
https://github.com/offensive-security/nethunter-app/releases

The cherry on the cake would be an option to install the latest release candidate.

Appreciate your thoughts.

[kimocoder]

Good idea.. I'm fixing the big PR now..