admin menu  spans all width and centers items

since we removed horizontalNav.js from the backend
we need a way to replicate the previous behavior
display: table; on the parent <ul> and using
display: table-cell; on chil <li> ensures we
always use the whole width of the parent container
and we span the children <li> accordingly using
the same width across the given space

Conflicts:
	backend/app/views/spree/layouts/admin.html.erb


Same as #3725 fixes #3720 on master