Implemented "cancel" for edit

src/scenes/DocumentEdit/DocumentEdit.js

@@ -1,5 +1,6 @@
 import React, { Component } from 'react';
 import { observer } from 'mobx-react';
+import { browserHistory } from 'react-router';
 
 import store from './DocumentEditStore';
 
@@ -50,6 +51,10 @@ class DocumentEdit extends Component {
     }
   }
 
+  onCancel = () => {
+    browserHistory.goBack();
+  }
+
   state = {
     scrollTop: 0,
   }
@@ -85,6 +90,9 @@ class DocumentEdit extends Component {
           <MenuItem onClick={ this.onPreviewToggle }>
             Preview <Switch checked={ store.preview } />
           </MenuItem>
+          <MenuItem onClick={ this.onCancel }>
+            Cancel
+          </MenuItem>
         </DropdownMenu>
       </Flex>
     );