[ci] Visual regression testing using backstopjs

[kristofferjs]

• Dedicated studio
• Chrome headless is running in docker for exact render across platforms

For designers:

npm backstop-test-studio to view studio and edit data
npm test:backstop (while studio is running) to test local changes. (docker required)
npm test:backstop:approve to approve visual changes

[rexxars]

👍

[kristofferjs]

Did som changes before we merge

• Moved the backstop folder to test
• No need to run backstop-test-studio before running test (it starts and quits on npm run test:backstop )

[mariuslundgard]

Hm, when I run npm run test:backstop I get

@sanity/core not installed in current project
Project-specific commands not available until you run `sanity install`
"start" is not available outside of a Sanity project context.
Run the command again within a Sanity project directory, where "@sanity/core"
is installed as a dependency.

[kristofferjs]

@mariuslundgard Did you run npm install and npm run bootstrap first?

[mariuslundgard]

Yes, it works! (6 tests failing)

[mariuslundgard]

Is this script still used or can it be removed?

[mariuslundgard]

Ah, it's used for updating content.

PS. Would be nice to document how to use these tests in the root README as part of this PR.