chore: move CI builds from travis-ci.org to GitHub actions

[vlukashov]

Description

The CI builds cover the following use cases:

• run a full set of automated code checks for each PR (see unit-tests.yml and visual-tests.yml in .github/workflows/)
  • when a new PR is open (for any branch having these workflow .yml files)
  • 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

Type of change

• Bugfix
• Feature
• Internal change

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• I have added a description following the guideline.
• The issue is created in the corresponding repository and I have referenced it.
• I have added tests to ensure my change is effective and works as intended.
• New and existing tests are passing locally with my change.
• I have performed self-review and corrected misspellings.