Make request content type dropdowns accessible

[lornajane]

I think there might be more than one occurrence of this that belongs under #1279 but let's start with the content type dropdown on request samples.

Currently, it's a <div> tag rather than an input element (it is much prettier than a dropdown, true!), with onMouseDown() and onTouchEnd() events and it does have an aria tag in it, but it's not focusable. The Firefox accessibility tools can't tell that this should be clickable so it doesn't actually throw any errors in the interface, it just looks like a display div.

Let's swap this component out with something pretty AND usable by everyone to select the content type.