Use ordering_key when matching opinions

[mlissner]

When we're looking up citations, we return clusters. Clusters may have multiple sub-opinions, but our citation graph is between opinions, not clusters.

Therefore, we need a way of choosing which sub-opinion is the correct one. Currently, we do it in a bad way. We should change this to match based on ordering_key if present.

[mlissner]

We should probably do this one before the next run of the citation finder.