Navigating between projects can cause overwriting

[hallpell]

This is a bug that will sometimes cause students to overwrite work, which is recoverable but in an unintuitive way.

Steps to reproduce:

1. Save a project
2. Modify and don't save it (so it is in a dirty state)
3. Attempt to navigate to another project using the navigation menu in the top left
4. Hit cancel on the "Reload Page? Changes you made may not be saved" prompt
5. Hit save. This will overwrite the project you were trying to navigate to with the current version of the project you are leaving.

This is happening because attempting to navigate to a new project will always change the hash, and hitting cancel on the prompt doesn't undo that. So when you hit save (which is a natural response in the above workflow), it saves in the destination project, even though we still have the old code.

My initial investigations into this led me to onbeforeunload, which seems like it should be controlling what we're interested in...