Add a submit handler function for our form

This is still a stub that doesn't actually _do_ aything yet.

Note that we're using the "uncontrolled components" React pattern for
the input field here as described here:

	https://reactjs.org/docs/uncontrolled-components.html

The alternative, forcing a new call to React's setState on every
keypress seems really silly to me, (I don't care to have React getting
involved on every keypress).