Add standard import map tag to base template.

Change-Id: I3a796978039a979de8c1fd5ff3d2f92dedc5979c
diff --git a/src/main/resources/templates/base.html b/src/main/resources/templates/base.html
index 598c6c8..4c45403 100644
--- a/src/main/resources/templates/base.html
+++ b/src/main/resources/templates/base.html
@@ -6,6 +6,7 @@
   <title>{#insert title}{/}</title>
 
   <link rel="stylesheet" type="text/css" href="/base.css" />
+  <script type="importmap" src="/web_modules/import-map.json"></script>
   <script type="module" src="/components.js"></script>
 
   {#insert head}{/}