Show/hide extension button based on URL query

To show the "Add Extension" button, you must now add `?extensions` to
the URL used to load the GUI.

src/components/gui/gui.css

@@ -159,3 +159,7 @@
     border: 1px solid lightgray;
     background: white;
 }
+
+.hidden {
+    display: none;
+}

src/components/gui/gui.jsx

@@ -47,6 +47,9 @@ const GUIComponent = props => {
         );
     }
 
+    const enableExtensions = window.location.search.includes('extensions');
+    const extensionButtonClasses = `${styles.extensionButton} ${enableExtensions ? '' : styles.hidden}`;
+
     const tabClassNames = {
         tabs: styles.tabs,
         tab: classNames(tabStyles.reactTabsTab, styles.tab),
@@ -94,7 +97,7 @@ const GUIComponent = props => {
                                     />
                                 </Box>
                                 <IconButton
-                                    className={styles.extensionButton}
+                                    className={extensionButtonClasses}
                                     img={addExtensionIcon}
                                     title={addExtensionMessage}
                                     onClick={onAddExtensionClick}