Sandboxed code execution and ability to make HTTP requests

• feat(eval): safe/sandboxed execution of student code thanks to docker
• feat(eval): provide _xhr() method to make http calls from student code
  => BREAKING CHANGE: evaluator will not timeout anymore => _setTimeoutMessage() is removed
• fix: log parsing status to stderr instead of stdout
• fix: include runtime errors (e.g. uncaught promise rejections) in logs
• fix: prevent invalid characters in student output files