Added close buttons to data-import status notices in the start page. These buttons are non-functional for the moment.
Added a DataImportsController#update method and route.

It will be invoked when the user clicks on the "x" (close) button of the data-import-status alert. It will set the "show_alert" attribute of the DataImport instance to false, which will cause the alert to be hidden from that moment on (this is not yet implemented).