Move build script into Celery task

[deoktr]

Move build script into a Celery task that will be started directly from Django and executed on a worker.

TODO

• Move build script into dhost/builds/tasks.py
• Get the result of the build into the Django ORM (call Build.end_build())
• Manage errors, in case of problems with the build, also ensure that the build is done

Extra

• Get a live output of the process execution through the API with a websocket