Warn user if they may lose changes by navigating away #27

If a user clicks a "documentation" link for a competency, by default it will open in the current window and they will lose their progress.

I don't like the idea of using window targets and opening in a new window, since most users should be familiar with their browser to "open in a new tab" with right click.

But if unsaved changes may get wiped out maybe we warn the user before navigating away.