Add w.js when wc_tracker is enabled.

[timmyc]

For #8

In preparation for releasing wc-admin to the .org repository, we should begin to add in tracking to aspects of the project so we can get a better idea of how users are interacting with the new designs. To enable us to do so, this PR adds a script tag for w.js to be loaded onto wc-admin pages when the option to woocommerce_allow_tracking is set to yes. w.js will give us the necessary pipeline to record tracks events.

My first inclination was to use wp_enqueue_script but it appears there is no way to easily defer/async load the stats asset this route, so instead I took some inspiration from loadScript in Calypso to arrive at this solution. Since I can't think of an immediate need to load other external scripts like this, I figured doing a quick one-off implementation would be good for now, but would absolutely be open to ways to improve this interesting code 😄 .

Questions

• Not entirely certain if tracks will record properly for sites that are not WooPack ( /cc @greenafrican and @psealock for any ideas here )
• In other projects when loading w.js a cache buster string is added to force a download of a new version. I have omitted that here, but we will likely need to add that in 😢

After this is merged in, I anticipate doing another PR to create a simple lib/analytics wrapper that can key off of wcSettings.trackingEnabled that will allow components to record tracks. At this layer we can also add in logic to try and filter out our test envs from being tracked too.

To Test

• Enable tracking on your site by updating via update_option( 'woocommerce_allow_tracking', 'yes' );
• Open up a wc-admin page, and in the js console, verify that _tkq is present
• Disable the wc tracker update_option( 'woocommerce_allow_tracking', 'no' );
• Refresh the wc-admin page and verify _tkq is not present on the window

[psealock]

Not entirely certain if tracks will record properly for sites that are not WooPack

Events will be recorded correctly, but users will be tracked by a cookie-based anonymous id. In terms of measuring usage, I think this will be sufficient.

[psealock]

Working as described. w.js loads and records initial event

[timmyc]

Thank you kindly for the feedback and confirmation on the non-Jetpack sites @psealock. I'm hoping we don't need to worry about cache busting too often. Did that ever get improved from a systems side with an etag or some sort of automated expiry?