Warn users if the visual editor will modify their code, and offer to switch to the raw editor.

**WIP**

There aren't any tests yet, but I wanted to see what people thought about this approach. Currently, the confirmation is a javascript confirm dialog. Should it be a modal -- if so, where can I find that? What should the messaging be? Now, it's just a feeble: *Visual editing may cause data loss. Click cancel for raw editor.*

The problem I'm running into now is that tinymce is adding a `\ufffd` character surrounded by a hidden span. I don't know why, but it seems related to the `valid_elements` setting...

@cahrens @nasthagiri @andy-armstrong