Upload importance should be generated procedurally in Nextflow workflows

We also discussed how the `[upload-importance]` section in a graph config file should be generated instead of manually written by the user, at least in Nextflow workflow, since the plugin used, time of run, etc. are all generated by the process.

This is a low priority issue so I don't think we'll address it for a while, but I'm opening this issue to document it for now.