add default, opt-out Google Analytics script for collecting generic usage information

[cvan]

We want usage information on approximate MAU and DAU to be able to gauge interest and usage of the Unity WebVR Assets package released earlier this month.

Developers should be able to remove or configure this however they want. See #164 for documenting/messaging.

We have a Google Analytics 360 account set up already (ID: UA-77033033-6). It's injected by Unity on this project's Asset Store detail page.

@delapuente wrote a great docs page on configuring Google Analytics for developers.

This page describes how to use GA's gtag.js (scroll to the last few images of that section). Possibly we may want to use analytics.js instead.

We want to have a default system that does the following for users:

• respects the browser's Do-Not-Track feature (i.e., check navigator.doNotTrack === '1')
• includes an option to opt out of Mozilla's GA (i.e., add window['ga-disable-UA-77033033-6'] = true)

Here's a snippet of JavaScript to possibly work from.

Also, consider looking at these GA plugins (which are used on https://vr.mozilla.org/).

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Parent issue: #165
Related issues:

• collect default, opt-out basic performance timing data in Google Analytics (as custom GA events) #170 for adding perf data,
• Add a Help button (with icon and label text) and modal blurb to WebVR Template's on-page UI #167 for building the Help modal,
• Document Data Collection from telemetry services #164 for documenting/messaging