Migrate to CircleCI in ckeditor/ckeditor5-angular

[pomek]

Provide a description of the task

To finish the CKEditor 5 migration, we need to configure the CI.

• We do not need dynamic configs like we did in CKEditor 5.
• Coveralls need the COVERALLS_REPO_TOKEN environment variable (token).
• The Slack notifier needs the CKE5_GITHUB_TOKEN, CKE5_CIRCLE_TOKEN, and CKE5_SLACK_WEBHOOK_URL variables.
• To install the latest Chrome, use the ckeditor5-dev-ci-install-latest-chrome binary.
• To figure out steps to execute, follow the Travis configuration.
• Replace the badge in README.md.
• When everything works as expected, remove Travis files.
• Do not forget to introduce a nightly build. It can be added via config or the Triggers section. Let's execute it daily at 12AM (UTC).

[pomek]

Tips for implementations in another repositories:

• https://app.circleci.com/settings/project/github/ckeditor/ckeditor5-angular/advanced
  • Build forked PR, do not pass secrets to them.
  • Auto-cancel redundant workflows
• https://app.circleci.com/settings/project/github/ckeditor/ckeditor5-angular/triggers
  • Nightly: pass the isNightly variable
  • Run it daily, at 12 AM.