Add auton. mode selector to driver view website

[gerth2]

Add a control for "next" and "prev" to the driver view. Selecting these buttons should trigger a change in the autonomous mode on the robot.

Code must deal with the inherent asynchronousness of many clients sending prev/next commands, the web server processing those commands, and the autonomous routine updating from those commands.

Implementaiton must have a generic interface, as it is to be rolled into common libraries.