Bug 1054410: Add special tracking for clicks on Learn more for pocket_1.

Also adds the GA tags that jcollings added to prod to prevent copy-pasting over them.

This is currently live on snippets.allizom.org with a `SAMPLE_RATE` of `0.8`. You can test by using the [snippet-switcher](https://github.com/Osmose/snippet-switcher) and setting your URL to `https://snippets.allizom.org` and your name to `pocket`. You will see three snippets; this change affects the pocket snippet that shows a popup when you click "Learn more" and sends an impression to the stats server for that event.