Force display of the top-most recipient_bar's recipient_row_date
when the floating_recipient_bar is just about to overlap and
becomes hidden while user is scrolling.
Fixes#4844.
We now have specific HTML/CSS classes for message fading and
user fading. They currently both have the same effect, changing
opacity, but we can now more easily treat them differently.
This change also removes "faded" attributes in compose-fade,
which avoids some confusion related to landing pages having
a "faded" class as well.