Fix job submission

Various fixes and cleaning in the code handling job submissions:

• do not touch umask (job files in the datastore have the same uid as the django server)
• sanitise job file names to prevent a vulnerability
• always create the job dir, even if the job has no input files (fix #200)
• use the same code (the upload_data() helper) in the UI and the API
• fix a race condition (the redis notification was fired before the db commit)