t/devices.html: add device dropdown for adding actuator ID

[Trek Hopton]

We should add a dropdown menu for the device part of the actuator ID. Currently we add actuators via the VidGrind devices page. We can add an actuator to the site by setting its ID to any string. This is confusing because when the add button is clicked, nothing will show if the ID doesn’t start with a Device ID. Since I think we only want to add actuators that are associated with a device, I think we should provide a dropdown menu to select which device you want to start the actuator ID with.

At the very least a description should be provided to explain that the actuator ID needs to start with a device ID, or an error should be shown if the user inputs the wrong ID prefix.