check for duplicate last names in roster and adjust

[Benjamin Skinner]

When initializing a course, the program should check all last names for duplicates and append first names to create unique repos. To be fully robust, it could check for continued duplication (first and last names are the same) and append a number for unique values.

When adding students, last names should be checked against current student repos (use API) and the first name added to the new repo.