Atc move utils to common

in preparation for a larger move of code from gh-book into a common directory. `utils.coffee` and `path.js` are now used by both `gh-book` and `atc` so I moved them into a common directory.
For example, there is DnD code that uses path.js and is needed in `gh-book` but results effectively in a NoOp in `atc` (atc does not have relative paths)

# Testing

- only paths of coffee files changed
- [ ] book loads
- [ ] html file loads properly
- [ ] create a new HTML file in a book (to make sure the paths file still loads and works) and confirm it was saved
- [ ] create a copy of an HTML file (to make sure the paths file still loads and works)