Exported data from previous year cannot be imported

[psvenk]

Because previous year data do not include all of the same information from current year data, attempting to import data exported from the previous year throws errors in the import code. There also seem to be some errors relating to the absence of the "current" quarter in imported data; making our code more versatile not only fixes this but paves the way for doing away with special-casing "current quarter" in #82.

I have done a little bit of investigation toward a fix; some WIP code is in the fix-import branch (b758ab4) in case anyone wants to take a look. I've been following the strategy of adapting the import code to work with already exported data, but we should additionally make sure that exported files from the previous year include all of the fields that one would expect from a current-year export. Providing flexibility in the import code is still important because we don't want an export from 2.8.1 to be useless ("just export it from a newer version" isn't a valid excuse, given that Aspen makes old data inaccessible when rolling over to the new school year, and it breaks semver).