Unloading the client (eg. by deactivating the Hypothesis extension) does
not currently work properly in VitalSource ebooks or other scenarios involving
iframes. Add an "Unload client" toggle button in the VS test page at
http://localhost:3000/document/vitalsource-epub to help with testing
this.

The code for this button has been extracted from the dev server's home page into
a shared `controls.js` module.

In the process I removed the comments about Babel from util.js/index.js as these
are much less relevant now, and instead added some notes at the top of
dev-server/static/scripts/util.js about why that file is not an ES module.