Disable buttons until a scan is performed;

Closes #122 (closed). Toggle whether or not the two feedback buttons (Re-Scan and Confirm) are enabled depending on if there was a successful scan. Enable the buttons when the scanner is paused and disable them when it is not.

TESTING: Navigate to the guest visit scanner in the app (Guest Forms -> Guest Visit Check In (Local Database)) and make sure the buttons are disabled when the camera is activated and enabled when a bar-code formatted as Code 39 is successfully scanned. Also, make sure they are disabled after pressing the Re-Scan Barcode button.

NOTE: I also cleaned up some un-needed lines of code, and added some comments to explain how to use the scanner with different formats.