Allow for disabling JavaScript tracking code embeds without disabling REST API

It would be nice to be able to disable the JavaScript tag embedding without also disabling the REST API. Right now there is a just a global "Enabled/Disabled" kill switch. However, I want to be able to use the REST API portion of the code, but do not want to embed the JavaScript tag into every page.

I propose adding a new `intercomio_js_tracking_enabled` variable that defaults to TRUE in order to preserve existing functionality without breaking anyone's sites. If this is set to FALSE then we skip the code responsible for embedding the JS tags into the page.

This can then be disabled via the admin UI, or settings.php, or whatever other solution you might use.