Refactor: `Answer` -> `Comment`

[e-caste]

Any reply to a Question or to a reply of a Question, currently known as Answer, should be renamed to Comment instead.

This is because

1. the Question can contain a suggested type for the answer (e.g. essay) or a selection of proposed answers (e.g. multiple choice), and
2. the Comment can contain a proper answer, which is e.g. the user-selected choice of a multiple choice question or the essay (see #97).

The answer is what will be effectively considered in the simulations, while the comment can be left under a question or another comment as a reply.