Test: Reintroduce visual regression testing

We've recently did some groundwork for visual regression testing, but had to disable the tests to due resolution issues when running Electron locally compared to GitHub actions.
Since then we've switched to Chrome in our tests. As a result we should re-visit the visual regression tests.

Snapshot plugin is added here:
https://github.com/netlify/netlify-cms/blob/a91e8316f7d4618f2fa2a3d94bc733b5a5d3ade8/cypress/plugins/index.js#L14

We can control browser resolution here:
https://github.com/netlify/netlify-cms/blob/a91e8316f7d4618f2fa2a3d94bc733b5a5d3ade8/cypress/plugins/index.js#L168

And enable the snapshot testing here:
https://github.com/netlify/netlify-cms/blob/a91e8316f7d4618f2fa2a3d94bc733b5a5d3ade8/cypress/integration/common/media_library.js#L64

We should also consider switching to large media storage of the snapshot images.