Persist the Network mock data (#1218)

Summary:
Persist the Network mock data, so it can survive after switching the plugins

Closes https://github.com/facebook/flipper/issues/1206
## Changelog

- Persist the Network mock data
- Remove the pref-filled '/' when creating a mock route
Pull Request resolved: https://github.com/facebook/flipper/pull/1218

Test Plan: <img width="807" alt="Screenshot 2020-06-02 at 11 45 15 PM" src="https://user-images.githubusercontent.com/410850/83540614-27e95000-a52b-11ea-8fc8-d7ad1373e904.png">

Reviewed By: mweststrate

Differential Revision: D21863561

Pulled By: passy

fbshipit-source-id: 4706ede721c7990a6bcc0bfe51f41e80306ffac7

desktop/plugins/network/ManageMockResponsePanel.tsx

@@ -129,7 +129,13 @@ function RouteRow(props: {
         {props.showWarning && (
           <Icon name="caution-triangle" color={colors.yellow} />
         )}
-        <TextEllipsis>{props.text}</TextEllipsis>
+        {props.text.length === 0 ? (
+          <TextEllipsis style={{color: colors.blackAlpha50}}>
+            untitled
+          </TextEllipsis>
+        ) : (
+          <TextEllipsis>{props.text}</TextEllipsis>
+        )}
       </FlexRow>
       {showCloseButton && (
         <FlexRow onClick={props.handleRemoveId}>