Remove HTML Preview Tool from navigation and add to gitignore

- Removed HTML Preview Tool from navigation menu in Layout.js
- Cleaned up unused Code import
- Added HTML Preview related files to .gitignore
- Project builds successfully without HTML Preview Tool

.gitignore

@@ -8,6 +8,7 @@ node_modules/
 
 # Production
 /build
+/backup
 
 # Misc
 .DS_Store
@@ -21,6 +22,17 @@ npm-debug.log*
 yarn-debug.log*
 yarn-error.log*
 
+# HTML Preview Tool (abandoned)
+src/pages/HtmlPreviewTool.js
+src/pages/HtmlPreviewTool.js.backup
+src/pages/components/PreviewFrame*.js
+src/pages/components/ElementEditor.js
+src/pages/components/InspectorSidebar.js
+src/pages/components/Toolbar.js
+src/pages/components/CodeInputs.js
+src/pages/components/SimplePreviewFrame.js
+src/pages/components/PreviewServer.js
+
 # Runtime data
 pids
 *.pid