Markdown Support in Comments

These changes add the ability to *italicize*, **bold**, and `code` the text of comments (among other things) by parsing Markdown.

To keep the user experience consistent, a new field called `original_text` is added to the `Comment` model, which keeps track of the untouched user input. This column is `NULL` by default. If this field has a value, it will be sent from the `edit_comment` view, so that the Markdown parsing is transparent to the user, and the comment they edit is exactly what they entered.

This requires [a migration](../blob/ecd84211512cdafe9dcff02129fa721642e70e67/review/migrations/0005_auto__add_field_comment_original_text.py) to add the `original_text` column.