Use csp nonces for inline scripts and styles

app/templates/admin_template.html

@@ -19,7 +19,7 @@
   <link rel="stylesheet" media="print" href="{{ asset_url('stylesheets/print.css') }}" />
   {% block extra_stylesheets %}
   {% endblock %}
-  <style>
+  <style nonce="{{ csp_nonce() }}">
       .govuk-header__container { border-color: {{header_colour}} }
   </style>
   {% if g.hide_from_search_engines %}