Redesign contact list

This is what I had in mind for Briar GTK:

Source: https://gitlab.gnome.org/Teams/Design/os-mockups/-/blob/f69fa82f8676582d900af8716522a27ddd77d87f/mobile-shell/mobile-shell-convergence.png

briar-gtk#73

In particular I think we should at least show the latest message per chat.