Introduce CircleCI in cksource/mrgit

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 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).