Update the UX of the mobilebuild download

Summary:
Update the UX of the mobilebuild download. When one clicks download one can see the the installation in progress in the main screen. Once its downloaded, you can open the finder pointed to the folder of download.

I didn't write the test for InstallSection as I was not able to test it through react-testing library. Seems like the data gets added to the document little late and not instantly.

Next work:
- Search in sidebar
- Persist downloaded app data for a session
- Show download data in the form of progress bar.

Reviewed By: mweststrate

Differential Revision: D21709229

fbshipit-source-id: e67db68ff16155230becf86c913dc6b1ec9d482c

desktop/app/src/index.tsx

@@ -34,7 +34,7 @@ export {PluginClient, Props} from './plugin';
 export {default as Client} from './Client';
 export {MetricType} from './utils/exportMetrics';
 export {default as promiseTimeout} from './utils/promiseTimeout';
-export {clipboard} from 'electron';
+export {clipboard, remote, OpenDialogOptions} from 'electron';
 export {default as SupportRequestFormV2} from './fb-stubs/SupportRequestFormV2';
 export {default as constants} from './fb-stubs/constants';
 export {connect} from 'react-redux';

desktop/app/src/ui/components/colors.tsx

@@ -31,6 +31,7 @@ export const colors = {
   highlightTint70: '#c6d9ff', // Highlight - Tint 70 (illustrations only)
   highlightTint90: '#ecf2ff', // Highlight - Tint 90 (illustrations only)
   highlightBackground: '#edf2fa', // Highlight Background - Background fill for unread or highlighted states. Not intended for hover / pressed states
+  highlighButtonPrimaryColor: '#237FF1', // Blue color which is used in the button when its type is primary
   green: '#42b72a', // Green - Confirmation, success, commerce and status
   red: '#FC3A4B', // Red - Badges, error states
   redTint: '#FEF2F1',