This reworks the way that Hypothesis detects whether it is
already present on the page in embed.js, in a way that
enables it to report back the app.html URL for the
existing instance to the Chrome extension.

The Chrome extension can then update its state for that
tab to reflect the fact that it is not injected.

In this PR, that is done by simply setting the tab state
to inactive. We could in future add an indicator that
the user tried to activate H but that another instance
is already active, or have the extension override the
instance of H on the page.

 * Detect the <link> element added to the page by embed.js,
   rather than window.annotator. This enables detection to work
   when embed.js is run in an isolated world that shares the DOM
   but not the JS environment, as is the case for content scripts
   in Chrome and Firefox.

 * Change unloading to operate by firing an event at the <link>
   element. This enables a content script to trigger unloading
   of H from an isolated JS environment.

 * Change injection of embed.js in the Chrome extension to
   execute embed.js as a content script, rather than by
   adding it as a `<script>` tag. This enables embed.js to
   access extension APIs and also report its result back
   to the extension via the return value of the script.

   A side benefit is that it also avoids a bug in Firefox
   current WebExtensions implementation where `<script>` tags
   added to the page by a content script are not executed.