add questions-related models.  Improve templates.

Added Question, QuestionComment, Answer, and AnswerComment.  These
inheret from abstract classes that would be appropriate to use for other
models, like Discussion, Relpy (to discussion), Comment (on letter),
Letter, etc.

Re-organized the template inheretance structure to make it more
sensible.  Moved some things from general templates, where they didn't
belong, to specific ones.
Merge branch 'questions'