New Epiphany designs

Hi,

We have two sets of new designs for the future of Epiphany.

The first set is from Diogo Campos. These served as the basis for the 
new downloads button currently in master:

https://bugzilla.gnome.org/show_bug.cgi?id=755687
https://bugzilla.gnome.org/show_bug.cgi?id=755524
https://bugzilla.gnome.org/show_bug.cgi?id=755518
https://bugzilla.gnome.org/show_bug.cgi?id=755388
https://bugzilla.gnome.org/show_bug.cgi?id=755378
https://bugzilla.gnome.org/show_bug.cgi?id=755292

The second is by Allan Day: 
https://raw.githubusercontent.com/gnome-design-team/gnome-mockups/master/web/web-bookmarks-and-header-bar-wires.png

Please feel free to discuss here and in the bugs. At this point, it 
would be good to unify the two sets of designs and agree on what we 
would implement.

Michael