API and UI to delete and/or archive jobs

As @ViviYe has proposed, it would be nice to have a way to delete jobs. Some concrete steps could include:

- Add a `DELETE` method on job URLs that removes the job and all its files.
- Add a button to the UI that invokes this API method.
- Perhaps a second API that _just_ deletes the files but preserves the metadata (i.e., the JSON file describing the job and its log file).
- In a fancier version, provide a mechanism to delete a job's files while sending it to S3 or something for archiving.
- Maybe provide a way to do this in bulk to jobs older than a certain date?

(Expanded from #1.)