Disable Tails Installer options when there is no target USB stick available
When an available USB stick is removed, it's still possible to select one of the tails installer options:
A possible scenario that this could lead:
1. A user inserts an available USB stick.
2. He removes this USB stick and inserts another that could not be used (e.g: device is to small to install).
3. The user clicks 'Upgrade' nothing happens and the Tails Installer becomes unusable
issue