rename wagtail-theme to demo-theme

Author: Srishti-j18

bakerydemo/templates/base.html

@@ -32,7 +32,7 @@
             function updateThemeMode(event, { isInitial = false } = {}) {
                 const DARK = "dark";
                 const LIGHT = "light";
-                const STORAGE_KEY = "wagtail-theme";
+                const STORAGE_KEY = "demo-theme";
 
                 let currentMode;
                 let applyMode;