Remove Google Tag Manager from Wagtail page previews

[thibaudcolas]

Initially reported by @vossisboss. In base_page.html, we load Google Tag Manager if configured in the Django settings with no further conditions: https://github.com/wagtail/wagtail.org/blob/468ccd44a65dee3fa9f574bbdc72689fc3a220e1/wagtailio/project_styleguide/templates/patterns/base_page.html#L66-L82.

In production environments, this means GTM will be loaded for page previews by CMS users. This is problematic for privacy reasons, makes previews slower than they need to be, and adds noise in the analytics.

Instead, we should avoid loading GTM with a conditional on request.is_preview, and probably also is_pattern_library (though that’s not normally enabled in production so most likely not an issue).

Likely patch (untested):

{% if GOOGLE_TAG_MANAGER_ID and not request.is_preview and not is_pattern_library %}
        <!-- Google Tag Manager -->
        <script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
            new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
                                                      j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
                'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
        })(window,document,'script','dataLayer','{{ GOOGLE_TAG_MANAGER_ID|escapejs }}');
        </script>
        <!-- End Google Tag Manager -->
    {% endif %}

And:

{% if GOOGLE_TAG_MANAGER_ID and not request.is_preview and not is_pattern_library %}
        <noscript><iframe src="https://www.googletagmanager.com/ns.html?id=GTM-5XDDLH" height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript>
{% endif %}