add segment.io JS snippet for more analytic options

[robbkidd]

Optionally included when a Segment write key is set in the environment.
Disabled altogether if airgapped.

Renamed the _google_analytics partial so that it can collect whatever
other client-side analytics that may be added in the future.

Closes #1689

[robbkidd]

With SEGMENT_WRITE_KEY=justtestingdontmindme

Results in:

<script type="text/javascript" async="" src="http://cdn.segment.com/analytics.js/v1/justtestingdontmindme/analytics.min.js"></script>
<script>
  !function(){var analytics=window.analytics=window.analytics||[];if(!analytics.initialize)if(analytics.invoked)window.console&&console.error&&console.error("Segment snippet included twice.");else{analytics.invoked=!0;analytics.methods=["trackSubmit","trackClick","trackLink","trackForm","pageview","identify","reset","group","track","ready","alias","debug","page","once","off","on"];analytics.factory=function(t){return function(){var e=Array.prototype.slice.call(arguments);e.unshift(t);analytics.push(e);return analytics}};for(var t=0;t<analytics.methods.length;t++){var e=analytics.methods[t];analytics[e]=analytics.factory(e)}analytics.load=function(t){var e=document.createElement("script");e.type="text/javascript";e.async=!0;e.src=("https:"===document.location.protocol?"https://":"http://")+"cdn.segment.com/analytics.js/v1/"+t+"/analytics.min.js";var n=document.getElementsByTagName("script")[0];n.parentNode.insertBefore(e,n)};analytics.SNIPPET_VERSION="4.0.0";
  analytics.load("justtestingdontmindme");
  analytics.page();
    }}();
</script>

[TrevorBramble]

@hamburglar asked @chef/cia to take a look and... looks good to me?

[robbkidd]

@iennae I reckon we're approved for launch.