Use NCES/AI crosswalk when generating school code mappings

[drewsamnick]

We have a data set from the College Board that maps their school codes to NCES school ids. While we've found some issues with that data we would still like to use it in the cases where we cannot determine a better mapping. I've updated the scripts used to generate the school code mappings to

1. Also try to map all of the school codes found in the nces ai crosswalk file.
2. To merge the crosswalk data into the data generated by the scripts while maintaining any existing mappings in the database.

[drewsamnick]

Relax AP mapping