Add emergency stop button to interface about sights HOT 1 OPEN

The emergency stop button should be disabled rather than enabled by the config. This means it is always visible until the interface receives a config file that specifies otherwise.

How should we implement this? There are a number of different levels of control possible, from simply stopping servos (works for all robots) to cutting power (would require additional hardware).

What level of the code does this run at? We probably don't want it as part of the SIGHTS service, since it would have to be running in order to work. Supervisor is a good option, since it is (most likely) running if the interface is accessible. Implementing it with PHP is even lower, since the webserver is always running if the interface is accessible, allowing the button to work even if Supervisor is not available - but I don't think we need to go that far.

from sights.