MAGETWO-58031: Inserted Image on content block of Category references the admin URL

 - save directive instead of url into db

Author: Robert He

app/code/Magento/Catalog/view/adminhtml/ui_component/category_form.xml

@@ -193,6 +193,7 @@
                         <item name="add_variables" xsi:type="boolean">false</item>
                         <item name="add_widgets" xsi:type="boolean">false</item>
                         <item name="add_images" xsi:type="boolean">false</item>
+                        <item name="add_directives" xsi:type="boolean">true</item>
                     </item>
                     <item name="source" xsi:type="string">category</item>
                 </item>