Fix admin menu spacing

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

Fixes #3839

Conflicts:
	backend/app/views/spree/layouts/admin.html.erb