Feature/instructor import DO NOT MERGE  

This PR implements basic excel import functionality so that instructors can import an excel spreadsheet. The rows in the spreadsheet are cards and the columns are fields. In order to import the spreadsheet into a deck, the instructor must select or create a _card template_ and map the columns in the spreadsheet to the fields in the _card template_. Then the rows can be processed and turned into cards.

WORK IN PROGRESS -- DO NOT MERGE