Consider comment support

[Helen Hou-Sandi]

Comments don't actually have revisions, which is its own problem, but in the meantime it seems like storing a note when saving changes to a comment could be quite useful. In particular, I am thinking about Make/Core, where our general practice is to edit objectionable comments instead of delete them, but don't really have a central way to store what the content used to be and why it was changed.