Soften visually jarring greyout when autoreloading

This change detects when Shiny is autoreloading, and instead of
showing the typical disconnection treatment (dark grey), it
initially shows nothing and then quickly fades to very light
grey.

The goal is for instant autoreloads to feel seamless, but longer
autoreloads to make it clear when the autoreload has completed,
and still not feel as jarring as a full disconnect.

Author: jcheng5