CYB-121 Fix student import

ActiveRecord::Base #find_or_create_by must match on all passed in parameters for it to find, otherwise it will try to create.  So if you have an existing record with a unique key, but with other parameters that differ, it won't find the existing record, and will try create a new one, hence violating the uniqueness constraint of the database.  The existing code will blow up if a student already in the DataCultures system's record changes in Canvas.

Affects [CYB-121](https://jira.ets.berkeley.edu/jira/browse/CYB-121)