Fix visibility of the upload diff UI.
Review Request #10436 — Created Feb. 27, 2019 and submitted
The newer version of jQuery we're using on release-4.0.x has a behavior
.show()where it won't override a default style of
display: none. This meant that the way we were showing/hiding the
various pages of the upload diff UI (used both for the "Update Diff" and
"New Review Request" flows) wasn't working.
This change makes it so we don't have any default visibility and instead
Loaded the New Review Request and Update Diff UIs and saw the correct