Use "list" icon for all-messages, matching web's new icon there

We currently use a "globe" icon for the button on the main screen that takes you to the all-messages view. This has never been a great icon to use there, but we didn't feel we had a better one -- the web app used a "home" icon, and that seemed like it would put too much emphasis on that view. (Unlike on web, the all-messages view has never been the main view that we present.)

But now web uses a "list" icon:

That seems perfectly reasonable, and it's helpful to have the same icon on web as on mobile. So let's switch to that one.

(From chat thread here.)