Update components to include promises
Infrastructure for AJAX requests returning ES6 promises
Use promises for GET requests
Add es6-promise to package_data
All aboard the promise train
Return JSON from contents API checkpoint methods
Semicolons
Re-raise errors with throw instead of Promise.reject()
Update JS docstrings in contents API
Load promises polyfill from a script tag

So all JS code can assume promises work, without needing to require it.
Merge pull request #6924 from takluyver/contents-promises

Use promises in the JS contents API