EditObjectController now exits edit mode when it is destroyed.
It also injects a check function in the navigation service to
replace the old functionality implemented in EditNavigationPolicy.
https://github.com/nasa/openmct/issues/1360
Remove mct-before-unload, and move the functionality to the navigation
service. The navigation service considers "unload" to be a navigation
event and prompts in much the same way as it would before any other
navigation event.
https://github.com/nasa/openmct/issues/1360