Add linter to avoid stuff like console.log in pull requests and code

[pinussilvestrus]

Actually we have this linting option enabled: https://github.com/schul-cloud/schulcloud-server/blob/master/.eslintrc.json#L15

This automatically enables the no-console option (https://eslint.org/docs/rules/no-console). Travis just fails when there is a console log. So this can be closed.