Analytics: Add utm URL params to page view event

[mattm]

This PR adds utm tracking support to Calypso by adding each utm query parameter as an event property for the calypso_page_view event. This will enable us to point ads to Calypso and be able to measure their impact on user behavior broken down by source, campaign, medium, etc.

To test:

1. Turn on debugging for the Analytics module:

localStorage.setItem('debug', 'calypso:analytics');

2. Go to http://calypso.localhost:3000/plans. Verify there is a debug message with:

Record event "calypso_page_view" called with props {path: "/plans"}

3. Go to http://calypso.localhost:3000/plans?utm_campaign=email. Debug message should now contain props: {path: "/plans", utm_campaign: "email"}

4. Go to http://calypso.localhost:3000/plans?test=derek. Debug message should not include this parameter because it doesn't begin with utm_. Props should just be {path: "/plans"}.

Test live: https://calypso.live/?branch=add-utm-param-tracking

[dbspringer]

Looks good! 🚢

At some point we probably ought to es6-ize this file, but that's a separate project.