Introduce Sandy welcome dialog

Summary: Added a welcome screen so that our users won't feel alienated / overwhelmed by the new Sandy design. With this in place we can start opening the GK to a subset of FB employees in the next release

Reviewed By: passy

Differential Revision: D25088630

fbshipit-source-id: 7337d670928f048a2c2fa60d98ac4bdcefb67c4d

desktop/app/src/chrome/SettingsSheet.tsx

@@ -247,7 +247,7 @@ class SettingsSheet extends Component<Props, State> {
         />
         {!disableSandy && (
           <ToggledSection
-            label="Enable dark theme"
+            label="Enable dark theme (experimental)"
             toggled={darkMode}
             onChange={(enabled) => {
               this.setState((prevState) => ({