[WIP] Feedback tests

[kcranston]

This is a start on the feedback tests. Couple of notes:

• I refactored the single feedback method into two separate methods for ease of testing (and to avoid testing the argparse argument)
• The tests themselves need to be fleshed out, and we need to cover edge cases (missing student repos, repo not initialized, etc, etc)
• The code here does not take into account any of the proposed changes from Clone and move different file types and clean up copy assignment files & fix code cov #295 or scrub hidden tests from notebooks using nbclean #290