Fixed sandbox. Multiple tabs work now.

WiP on selectable mosaico templates.

TODO: Make files always point to trusted URL, such that we don't have to rebase them. They are public anyway. The same goes for mosaico endpoints: /mosaico/templates and /mosaico/img

client/src/lists/segments/List.js

@@ -47,7 +47,7 @@ export default class List extends Component {
 
                 <Title>{t('Segment')}</Title>
 
-                <Table withHeader dataUrl={`/rest/segments-table/${this.props.list.id}`} columns={columns} />
+                <Table withHeader dataUrl={`rest/segments-table/${this.props.list.id}`} columns={columns} />
             </div>
         );
     }