Use meta tag to tell search engines not to index

Google’s documentation says:

> robots.txt is not a mechanism for keeping a web page out of Google. To
> keep a web page out of Google, you should use noindex directives

A noindex directive means adding the following meta tag to pages that
shouldn’t be indexed:
```html
<meta name="robots" content="noindex" />
```

It’s also possible to set the directive as a HTTP header, but this seems
trickier to achieve on a per-view basis in Flask.

I’ve implemented this as a decorator so it can quickly be added to any
other pages that we decide shouldn’t appear in search results.

app/templates/admin_template.html

@@ -17,6 +17,9 @@
   <style>
       .govuk-header__container { border-color: {{header_colour}} }
   </style>
+  {% if g.hide_from_search_engines %}
+    <meta name="robots" content="noindex" />
+  {% endif %}
   <meta name="google-site-verification" content="niWnSqImOWz6mVQTYqNb5tFK8HaKSB4b3ED4Z9gtUQ0" />
   {% block meta_format_detection %}
     <meta name="format-detection" content="telephone=no">

app/templates/views/support/public.html

@@ -3,7 +3,7 @@
 {% from "components/page-header.html" import page_header %}
 
 {% block per_page_title %}
-  The GOV.UK Notify team cannot give advice to members of the public
+  The GOV.UK Notify service is for people who work in the government
 {% endblock %}
 
 {% block maincolumn_content %}