[server] refactor cleanup() code used by "cubicweb-ctl delete" Try to make each logical step (i.e. "delete database" and "delete user") a bit more visible and explicit. This is a preparatory work to easily plug the "delete namespace" feature.

[views] visual refactorings (closes #3803685) - pageContent: - remove the shadow - add a light border - slightly round the corners - left column: - ensure width make it aligned with the logo (header) - ensure the top border of the first box is aligned with the pageContent div - do not use serif font for box headers