Don't fail when submitting a duplicate job

[cjordan]

(I guess this is similar to #28)

The newest version of the ASVO web interface returns

{"error": "Job already queued, processing or complete."}

when a job has... already been submitted. This isn't what the server used to do, and I think it's overall better to have the server simply return the job ID corresponding to the earlier-submitted job. Thoughts?