Make CI work with GitHub Actions
Created by: nejch
This will need two secrets configured by an admin in order for the DockerHub deploy to really work: DOCKER_USERNAME and DOCKER_PASSWORD, to replace encrypted secrets previously committed in the .travis.yml config.
This probably also needs an additional approval for every first contribution (see: https://github.blog/changelog/2021-04-22-github-actions-maintainers-must-approve-first-time-contributor-workflow-runs/) but here is an example run from my fork: https://github.com/nejch/marge-bot/actions/runs/833323869
Helps with https://github.com/smarkets/marge-bot/pull/283 not running CI