Add job cancel / delete route

[sverhoeven]

As a someone who submitted a job to the web service I would like to get rid of the job.
Job can be in different states:

• Cancel a queued job -> remove job from scheduler + remove job directory + remove job from db
• Stop a running job -> remove job from scheduler + remove job directory + remove job from db
• Cleanup a completed job -> remove job directory + remove job from db

This would entail creating DELETE /api/job/{jobid} route.

Instead of removing job completely we could mark job as cancelled in db and retain the job directory.

Do we want to distinguish between cancelling a job in the scheduler or deleting it from the web service? Create route to cancel job and a route to delete job.