rework notes to have a body and author

[gravitystorm]

The current system is to have notes, and create a comment when the note is created. This is fiddly, and leaves e.g. the tests convoluted and lots of hidden dependencies throughout the app code expecting that first comment.

It would make more sense for the note to have a body, instead of implied necessity of the initial comment. I've no idea why it wasn't like this to start.

There's also weirdness around the note having no user_id, but instead the note.author coming from note.comments.first.author. Given the first comment is again treated as something special, the ownership of the note should be on the note itself.

However, cleaning this up would involve a hairy migration, to convert the contents of the first comment on every note into attributes of the note itself. So it's not a high priority.