New welcome page for 3.38
This branch adds a new welcome page as the first step, before the language chooser.
The mockups for this work are here: https://gitlab.gnome.org/Teams/Design/os-mockups/blob/master/initial-setup/welcome-page-aday.png
Here is how the new page looks: 
And here is the adapted language page: 
Fixes: #7 (closed)
Edited by Philip Withnall