fix(Events): Duplicate node entered and node exited events

Changes

Duplicate events are no longer saved
Removed nodeEntered and nodeExited event saving in NodeComponent. nodeEntered and nodeExited events are only saved in the VLEComponent now.
When the student changes step, we now send the nodeExited and nodeEntered events in a single request to prevent the events being saved out of order
We now save a nodeExited event on window:beforeunload so that the nodeExited event is saved when the student refreshes the page or closes the tab

Make sure duplicate nodeEntered and nodeExited events are not saved when viewing the student project view
Make sure the nodeExited and nodeEntered events are saved in the correct order in the database when the student changes step
Make sure the nodeExited event is saved when refreshing the page or closing the tab
Make sure the nodeEntered event is saved when the student launches the unit

Closes #1873