git-autograder v3

[woojiahao]

Closes #12 and #11

There were many breaking changes introduced in this version, mostly because we have managed to establish a small sample of autograding so we are able to properly generalize and modularize autograding behavior.

Changelog

1. Moving from individual helper functions embedded within GitAutograderRepo to separate *Helper classes: repo.branches.branch() vs repo.get_branch()
2. Introducing custom wrapper classes around Remote, Commit, and Branch to encompass any necessary checks
3. Improve testing utility by including GitAutograderTestLoader to reduce # type: ignore comments in unit tests
4. Provide solution file test validation to reduce duplication of answer validation
5. Move imports to __init__ to clean up imports for users