Add a reload button to the editor

[JamesWrigley]

The context file can be modified simultaneously by multiple people either through the GUIs or through e.g. Jupyterhub or a terminal editor, and when this happens there's a danger of a user with an old version of the context file accidentally saving the old version. Ignoring the can of worms around merge conflicts, we should at least add a reload button to the editor and notify the user that the file has changed. Partially implemented in e119354.

[JamesWrigley]

Closing because this was merged in #119.