Ensure that all links, script and stylesheet elements added to the page
by the boot script are removed when the client is unloaded.

This fixes a problem where the recently added `<link>` for the notebook
app was not removed when the page was unloaded, causing attempts to
activate the browser extension on a page where the client had previously
been loaded and then removed, to fail.

The implementation works by tagging all elements with a
`hypothesis-asset` attribute in the boot script and then removing these
elements when the client is unloaded.