UI tweak: disable prioritization

This is probably not super important... but Michael N. mentioned today that for the simplest use cases, people already know approximately where they want to design, and won't be interested in the prioritization tool at all.

Instead of having a "Prioritization: None" dropdown option, we could have the entire "prioritization section" be a collapsable section.

If we find that this is in fact *most* user, we could have it unchecked by default and save a little vertical space.

Again, this doesn't seem very urgent compared to some of the other changes we want to make, but wanted to get it down.