In a non interleaved view when composing a message to another
conversation we fade messages which the user is not replying to,
to reduce the chance they send a message to a recipient they didn't
intend to. Also, it reduces the visual/cognitive processing required
to figure out where their message is going to go.
But, it's not necessarily clear to users that what the
fading means, so this commit adds a one-time compose banner
to explain what's going on the first time this comes up.
Fixes part of #29076.
Prakhar Pratyush
Anders Kaseorg