Use pincites to select correct sub-opinion when matching

[mlissner]

If we have a pincite like this: Roe at 442, we should use the 442 to identify the correct sub-opinion within Roe, rather than just blindly selecting whichever is first by ordering_key.

This is hard because we don't have page ranges for opinions, so we'll probably need to start by adding those to the model.

But ideally, imagine we have a cluster like this that's cited as "Roe at 457":

• Roe v. Wade (33 U.S. 442):
  • Sub-opinion1: (pages 442-456)
  • Sub-opinion2: (pages 456-466)
  • Sub-opinion3: (pages 466-470)

If that's the case, our citation to "Roe at 457" should go to Sub-opinion2, because that's where the correct page is.

[mlissner]

This is not important before our next run of the citation finder. Too difficult.