feat(frontend): add ability to download dashboard and chart as image (#9819)

* feat(frontend): add ability to export dashboard and chart as image (#6973)

* refactor: migrate download image as to typescript

* feature(frontend): download as image updates

add toast message if error occurs.
make generate file stem an internal method.

* refactor(frontend): move default background color to a const

* feat(frontend): wrap download image toast in translate function

superset-frontend/src/dashboard/components/HeaderActionsDropdown.jsx

@@ -28,6 +28,7 @@ import SaveModal from './SaveModal';
 import injectCustomCss from '../util/injectCustomCss';
 import { SAVE_TYPE_NEWDASHBOARD } from '../util/constants';
 import URLShortLinkModal from '../../components/URLShortLinkModal';
+import downloadAsImage from '../util/downloadAsImage';
 import getDashboardUrl from '../util/getDashboardUrl';
 import { getActiveFilters } from '../util/activeDashboardFilters';
 
@@ -217,6 +218,12 @@ class HeaderActionsDropdown extends React.PureComponent {
             onChange={this.changeCss}
           />
         )}
+
+        {!editMode && (
+          <MenuItem onClick={downloadAsImage('.dashboard', dashboardTitle)}>
+            {t('Download as image')}
+          </MenuItem>
+        )}
       </DropdownButton>
     );
   }