MDL-57338 templates: fix missing example variables

From Davids review, thanks

lib/templates/action_menu_link.mustache

@@ -23,7 +23,9 @@
     {
         "text": "Example link text",
         "showtext": true,
-        "url": "http://example.com/link"
+        "url": "http://example.com/link",
+        "classes": "menu-action",
+        "instance": "1"
     }
 }}
 {{^disabled}}

lib/templates/login.mustache

@@ -35,7 +35,8 @@
         "rememberusername": true,
         "passwordautocomplete": false,
         "signupurl": "http://localhost/stable_master/login/signup.php",
-        "cookieshelpiconformatted": ""
+        "cookieshelpiconformatted": "",
+        "username": ""
     }
 }}
 {{#hasinstructions}}

theme/boost/templates/core/dataformat_selector.mustache

@@ -30,10 +30,12 @@
     {
         "base": "http://example.org/",
         "name": "test",
+        "value": "test",
         "label": "Download table data as",
         "params": false,
         "options": [{"label": "CSV", "name": "csv"}, {"label": "Excel", "name": "excel"}],
-        "submit": "Download"
+        "submit": "Download",
+        "sesskey": ""
     }
 }}
 <form method="get" action="{{base}}" class="dataformatselector m-a-1">