Add automated site previews to pull requests for reviewing changes before deploying to live

[andrewsardone]

Every pull request should build a version of the site at a new URL for publicly previewing the site before merging in the change. This would allow us to test and catch errors, as well as iterate on site redesigns.

Deploying to something like Netlify would allow us to generate a new URL automatically on every pull request change, as described in this GitHub Action: https://github.com/nwtgck/actions-netlify