Reorganize landing page for new users
See merge request gitlab-org/gitlab-ce!15191
(cherry picked from commit 6ffb9ffd)
0a7a9bbd Reorganize landing page for new users
6377821a Add mobile styles
250d606a Update new project icon
f014fce1 Add four blocks to each welcome page
5c909f2b Add new icon; update content
95ab3bf5 Use flexbox instead of bootstrap columns
