Clicking Begin Training multiple times before first training is finished makes multiple jobs

[manthey]

I started finding superpixels on a bunch of images. After one finished, I labeled some superpixels and then clicked Begin Training. Initially this failed because it didn't have a jobId. I reload the web page and the clicked Begin Training, which queued a job. I could then click Begin Training many times, each one queuing another job. Ideally, if we have one job in the queue which hasn't started yet, we shouldn't queue another job.