Add bin/percy script for generating snapshots

[tannerwelsh]

What does this PR do?

Adds a helper script to allow us to generate snapshots for visual diff testing using Percy.io.

Now when a PR is ready for review, we can run $ ./bin/percy and it will build the site and generate snapshots for us.

Where should the reviewer start?

README

How should this be manually tested?

For an open PR, try running ./bin/percy and check out the snapshots created!

Any background context you want to provide?

What are the relevant tickets?

In Notion.

Screenshots (if appropriate)

Any other deploy steps?

Nope

[tannerwelsh]

Does this: mean that it also checks what the UI looks like in various browsers?

It sure does!