How do I change the page titles in the head section for each page? I would like to show: 'page title | site name'
For Magento 2.2 this is now in Content -> Design -> Configuration Then click edit on either website, store or view. Finally, go to the html head section and fill out Prefix or Suffix.
In 2.1, this is now handled by UI component and it's located under Content > Configuration > > Other Settings and then HTML Head section and here you can add a Title suffix
To do that, you need to go to the back Stores > Configuration > Design tab then you open the HTML Head section and here you can add a Title suffix. In your case it would be
| site name