Try faster fade-out on mark-as-read button

[gnprice]

This is a followup to:

• msglist: Leave blank space for "mark as read" button so using it doesn't cause messages to shift #562
• msglist: Leave blank space for "mark as read" button so using it doesn't cause messages to shift #565

See chat thread.

[alya]

I've been noticing this one when I use the app.

[gnprice]

Specifically, per Vlad's suggestion yesterday:

• Let's have it fade out in 100ms, rather than 2000ms.
• Let's also try having the animation scale the button down, as well as reducing opacity.

In the future it would be good to have the button respond optimistically, without waiting for an ack from the server. (I'll file an issue for that. → #685) Then we can make the animation longer again, because there won't be the built-in delay from waiting for the server.