The CI builds cover the following use cases:
 - run a full set of automated code checks for each PR (see the `Full QA` workflow in `.github/workflows/full-qa.yml`)
   - when a new PR is open (for any target branch)
   - when an open PR gets a new commit (including a force push)
 - do not run automated checks if the commit title includes `skip ci`
   The automated checks are skipped only for that one commit which has `skip ci`. For consequent commits that do not have `skip ci` the checks are executed normally.

The full set of automated tests includes both unit and visual tests:
 - Unit tests are executed with the Polymer 2 source code, and also with the Polymer 3 source code after running the P2-to-P3 conversion script.
   - The unit tests with the Polymer 3 source code are executed in a set of different browsers / platforms (via SauceLabs).
   - The unit tests with the Polymer 2 source code are executed in a few browsers running headlessly in the CI agent.
 - Visual tests are executed with the Polymer 2 source code in Chrome and Firefox (via SauceLabs).

Related to: https://github.com/vaadin/components-team-tasks/issues/586