Better support for cypress parallelization

[tzimmermann]

We're trying SauceVisual now on a bigger scale in our project.
It has become quite clear that the parallelisation of our Cypress test suite does not play well together with Sauce Labs Visual yet:

Problem:

• All test runs get their individual item in Sauce Labs Visual UI. We have 11x parallelisation of our test suite in CircleCI, thus the list gets super cluttered.

Potential solution:

• Aggregate all snapshots uploaded to one branch (or one PR) to the same line. Potentially, this would need a "finalise" step to be triggered from CI once all tests are finished (percy.io does it like that).

Slightly off topic:

• For better GitHub integration, it would also be nice to have a GitHub status check with a direct link to all the branch pipeline's snapshots.