Feature/instructor import

This PR adds the ability for instructors to upload/download a deck of cards from an excel spreadsheet. The instructor is expected to use a template spreadsheet so that the fields are parsed correctly when it is uploaded. The rows represent cards and the columns represent fields. The fields are derived from the *CardTemplate* that belongs to the *Collection*.

This PR does *not* add any buttons to the main collection interface, since that is currently in flux. Instead it adds the following orphaned views:

     /collection/<id>/download-template
     Downloads the spreadsheet template to populate with words.

     /collection/<id>/upload
     Form to upload the spreadsheet which will in turn create a deck of cards.

     /deck/<id>/download
     Downloads the deck of cards as a spreadsheet.

Todo:

- [x] Card template model(s) with initial data.
- [x] Spreadsheet template download.
- [x] Import spreadsheet into deck.
- [x] Export deck to spreadsheet.
- [x] Fix broken tests.

@jazahn Please review.