Fix warning for unsaved work on beforeUnload

Review Request #485 - Created March 17, 2017 and updated

Review Board
Previously, when editing a review or comments as a user without edit rights,
warning users about losing unsaved work did not get displayed.

Now the ReviewRequestEditorView's beforeUnload event handler does not check a
user's edit rights and only checks to see if any fields are being edited at the

New unit tests testing the onBeforeUnload event handler has been added.

Testing Done:
- Ran JS unit tests.
- Check that ReviewDialogView for both edit and non-edit privileged users are
  warned before unloading.

Bugs closed: 4084

Reviewed at