Unifying the prop names between app.params and the params used for
generating URls makes life significantly easier for everyone.
The generateUrl params schema referred to the change number in two
different ways, `id` and `changeId`. Modifying both instances to
`changeNum` helps maintain consistency within the app.
Bug: Issue 6708
Change-Id: I70b11f4e4336bb92f9c9062c69ee67613528f724