Visual testing: add Percy

Created by: chrismwendt

This adds visual regression testing to our e2e tests. I was surprised by how easy this was to set up! This will help prevent bugs (see list at the bottom).

Links:

• Percy: https://percy.io/
• Percy + Puppeteer docs: https://docs.percy.io/docs/puppeteer
• Sample build: https://percy.io/Sourcegraph/Sourcegraph/builds/1522732

Visual testing process:

• In CI, we run env PERCY_TOKEN=... percy exec -- yarn run test-e2e, which starts the Percy agent that knows how to upload images to percy.io
• In a test, we call percySnapshot('Some screenshot name') from @percy/puppeteer
• The Percy agent uploads all the images to percy.io
• The Buildkite build completes (visual regressions will not cause test failures in Buildkite, but will be added as another CI check requiring approval when there are differences)
• Percy checks the images:
  • If there are no differences, the Percy GitHub CI check goes green
  • If there are differences, the Percy GitHub CI check goes red, and someone must go to https://percy.io/Sourcegraph/Sourcegraph and approve the changes

Here's what it looks like:

UI bug list:

• https://sourcegraph.slack.com/archives/C0C324C91/p1551984503020000
• https://github.com/sourcegraph/sourcegraph/pull/2605