Add id=favicon in templates

The new dynamic favicon-changing code looks for an existing favicon in
the DOM with id 'favicon'. This uses that id in the templates, so we
don't end up with two favicon links in the page.

I was hoping that this would fix some issues I'm having with the favicon
getting stuck, but it doesn't seem to. :-(

notebook/templates/edit.html

@@ -2,7 +2,7 @@
 
 {% block title %}{{page_title}}{% endblock %}
 
-{% block favicon %}<link rel="shortcut icon" type="image/x-icon" href="{{static_url("base/images/favicon-file.ico") }}">{% endblock %}
+{% block favicon %}<link id="favicon" rel="shortcut icon" type="image/x-icon" href="{{static_url("base/images/favicon-file.ico") }}">{% endblock %}
 
 {% block stylesheet %}
 <link rel="stylesheet" href="{{ static_url('components/codemirror/lib/codemirror.css') }}">

notebook/templates/notebook.html

@@ -1,6 +1,6 @@
 {% extends "page.html" %}
 
-{% block favicon %}<link rel="shortcut icon" type="image/x-icon" href="{{static_url("base/images/favicon-notebook.ico") }}">{% endblock %}
+{% block favicon %}<link id="favicon" rel="shortcut icon" type="image/x-icon" href="{{static_url("base/images/favicon-notebook.ico") }}">{% endblock %}
 
 {% block stylesheet %}
 

notebook/templates/page.html

@@ -5,7 +5,7 @@
     <meta charset="utf-8">
 
     <title>{% block title %}Jupyter Notebook{% endblock %}</title>
-    {% block favicon %}<link rel="shortcut icon" type="image/x-icon" href="{{static_url("base/images/favicon.ico") }}">{% endblock %}
+    {% block favicon %}<link id="favicon" rel="shortcut icon" type="image/x-icon" href="{{static_url("base/images/favicon.ico") }}">{% endblock %}
     <meta http-equiv="X-UA-Compatible" content="IE=edge" />
     <link rel="stylesheet" href="{{static_url("components/jquery-ui/themes/smoothness/jquery-ui.min.css") }}" type="text/css" />
     <link rel="stylesheet" href="{{static_url("components/jquery-typeahead/dist/jquery.typeahead.min.css") }}" type="text/css" />

notebook/templates/terminal.html

@@ -2,7 +2,7 @@
 
 {% block title %}{{page_title}}{% endblock %}
 
-{% block favicon %}<link rel="shortcut icon" type="image/x-icon" href="{{static_url("base/images/favicon-terminal.ico") }}">{% endblock %}
+{% block favicon %}<link id="favicon" rel="shortcut icon" type="image/x-icon" href="{{static_url("base/images/favicon-terminal.ico") }}">{% endblock %}
 
 {% block bodyclasses %}terminal-app {{super()}}{% endblock %}