fix bug 775485 - welcome email tweaks

Couple of updates to welcome emails:
* Add more GA tracking and update 'utm_source' to 'developer.mozilla.org' so we show up correctly as a source in GA for other Mozilla properties like wiki.mozilla.org
* Update the `From:` email address to "Janet Swisher, MDN Community Manager" instead of "notifications"

Spot-check instructions:
* [x] [Add a `welcome_email` switch](https://developer-local.allizom.org/admin/waffle/switch/add/) set to active
* [x] Register a new account in [en-US](https://developer-local.allizom.org/en-US/)
  * You should see the welcome email output in the `foreman start` console output