test: set up Cypress visual diff smoke tests

[contolini]

Adds a global afterEach hook that runs after every test and takes a screenshot of the app's current state. Baseline screenshots are generated with yarn test-visual:baseline and comparisons against the baseline are made with yarn test-visual.

Changes

• Adds cypress-visual-regression library
• Adds a global Cypress afterEach hook

Testing

1. Run yarn test-visual:baseline to generate baseline screenshots.
2. Change some aspect of the app.
3. Run yarn test-visual to take new screenshots and compare against the baselines. It should fail if you changed something and pass if you didn't.

Notes

• I gitignored the entire screenshots directory. Normally you'd only ignore the actual subdirectory and check the base subdirectory into git. Because this is a new thing and all our environments are a little different, I decided to ignore everything for now.

[contolini]

This helper function wasn't working as expected with the Cypress global afterEach hook so I replaced it with Cypress' native request method.