[IMP] base_import: ControlPanelMixin and new design compatibility

- Use the ControlPanelMixin in the Import view;
- Adapt the stylesheet to fit with both community and enterprise editions;
- Convert the css into less, use mixins and variables defined in web;
- Return the reload() deferred in the on_reverse_breadcrumb callback so that the
  previous action waits to be properly reloaded before being shown.