Need a better UI for fixing up curation PRs with wrong information

[fossygirl]

In cases where a curator submits a proposed curation, then goes on to have discussion on the PR with the community, and then comes to understand their curation was wrong (but they now want to fix it), there is no UI affordance for them. Right now we're asking them to edit the YAML with a SPDX identifier and hoping they get it right.

They could close that PR and then open a new one with the correct information, but then you lose all the discussion and history about how they came to a different answer on the curation.

We should provide a UI for "oops, this is different now" that gives them the same affordances for new proposed curations.