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/users/List.js

@@ -53,7 +53,7 @@ export default class List extends Component {
 
                 <Title>{t('Users')}</Title>
 
-                <Table withHeader dataUrl="/rest/users-table" columns={columns} />
+                <Table withHeader dataUrl="rest/users-table" columns={columns} />
             </div>
         );
     }