(Server) Include Mock Component to Main Files

Summary:
- Add mock button if a client supports the function
- Open the dialog when clicking the button

Note:
- This is a part of this PR: https://github.com/facebook/flipper/pull/488

Reviewed By: mweststrate

Differential Revision: D20440145

fbshipit-source-id: 750099020e0b2d6ed10bb20e883f6b3be664ae79

desktop/plugins/network/RequestDetails.tsx

@@ -184,7 +184,9 @@ export default class RequestDetails extends Component<
             {response.headers.length > 0 ? (
               <Panel
                 key={'responseheaders'}
-                heading={'Response Headers'}
+                heading={
+                  response.isMock ? 'Response Body (Mock)' : 'Response Body'
+                }
                 floating={false}
                 padded={false}>
                 <HeaderInspector headers={response.headers} />