Stripping down the set of fields used in CSV upload

We normally only have names, email addresses, events, and roles when uploading - the other information recorded in `Person` (such as Twitter, GitHub, etc.) is usually only known for instructors, and can be added one-by-one after the fact.

To do: reuse an existing person if there's one in the database (match by email)

To do: make sure Task entries are unique (which means clearing them out during testing).