Clarify semantics of SchoolFilter to filter based on student's current school

[kevinrobinson]

This came up with the AttendanceImporter, as part of #1890, but it probably affects other importers as well.

New Bedford has a significant number of records that are skipped because it can't match the student local_id. This appears to be happening because of differences in how the Somerville and New Bedford export processes export the school_local_id field for attendance. Somerville does a join on the student, then the school (see the SQL). The NB Java export code appears to grab the school at the time that the attendance event occurred (rather than the student's school at export time, see the Java code). This means that for NB the "school filter" for Insights importer code don't work the same way they do for Somerville. For NB, the Insights importer would skip records for a student who is in a school we want to import now (ie, it would skip the records from when they were at their previous school).

One way to resolve this would be to remove these join fields from the export, and updating our importer code to look at the student's current school to do the filtering.