feat: create problem answer logs

[Tatehito]

Close #49

新たにProblemAnswerLogテーブルを追加し、問題解答時にログを保存するようにしました。

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[ykit00]

LGTM!

ProblemAnswerLog と UserSolvedProblem とで似たようなモデルになってしまったのでどうしようかちょっと悩みますねー