Use Scribe's undo manager

Use Scribe's undo manager. There are a couple of caveats, but those are due to how undo works in Scribe.

1. If you note a few things, and then undo, all of the notes you created will be undone. Not just the last one.
2. Undo only works within the current Scribe instance. (Would be nice to have some helpers in Scribe for operating on all instances on the page).

On a different note (pun intended), you might want to have a look at: https://github.com/guardian/scribe-plugin-noting/commit/c77441886825e0394a2e9ac00c4803ad8bda2466, as I accidentally committed that to master.