Notes

Oh boy. This adds notes functionality to iD, this addressing #1393 

Right now it takes the approach that:

* Notes have their own mode and ui
* Notes are in the history/graph and have the prefix 'e'
* In some places notes need to be explicitly excluded so they aren't dragged/selected/etc.

Todos:

* [ ] Re-add the show-notes toggle
* [ ] Tests
* [ ] Should this be able to create a new note? Ugh.
* [ ] The comment-posting UI 'lies' but that's probably all right.
* [ ] The styling could be cleaner
* [ ] We could use a notes icon instead of a blank circle as I'm doing now