Add 'only' to prevent action_url from leaking into header.html's context and creating a spurious action button

Have confirmed that this doesn't strip out any context variables that we do rely on - IndexView is the only thing descending from the generic base template which customises the header beyond title/subtitle/icon, and it does that by explicitly passing parameters to `include`, not by having them floating around in the template context.

wagtail/admin/templates/wagtailadmin/generic/base.html

@@ -5,7 +5,7 @@
 {% block content %}
 
     {% block header %}
-        {% include "wagtailadmin/shared/header.html" with title=page_title subtitle=page_subtitle icon=header_icon %}
+        {% include "wagtailadmin/shared/header.html" with title=page_title subtitle=page_subtitle icon=header_icon only %}
     {% endblock %}
 
     <div class="nice-padding">

wagtail/admin/templates/wagtailadmin/generic/index.html

@@ -6,9 +6,9 @@
 {% block content %}
     {% if can_add %}
         {% url view.add_url_name as add_link %}
-        {% include "wagtailadmin/shared/header.html" with title=page_title subtitle=page_subtitle action_url=add_link action_text=view.add_item_label icon=header_icon %}
+        {% include "wagtailadmin/shared/header.html" with title=page_title subtitle=page_subtitle action_url=add_link action_text=view.add_item_label icon=header_icon only %}
     {% else %}
-        {% include "wagtailadmin/shared/header.html" with title=page_title subtitle=page_subtitle icon=header_icon %}
+        {% include "wagtailadmin/shared/header.html" with title=page_title subtitle=page_subtitle icon=header_icon only %}
     {% endif %}
 
     {% block listing %}

wagtail/admin/templates/wagtailadmin/workflows/create.html

@@ -24,7 +24,7 @@
 {% endblock %}
 
 {% block content %}
-    {% include "wagtailadmin/shared/header.html" with title=view.page_title icon=view.header_icon merged=1 %}
+    {% include "wagtailadmin/shared/header.html" with title=view.page_title icon=view.header_icon merged=1 only %}
 
     <form action="{{ view.get_add_url }}" enctype="multipart/form-data" method="POST" novalidate>
         {% csrf_token %}

wagtail/users/templates/wagtailusers/groups/create.html

@@ -13,7 +13,7 @@
 {% block content %}
 
     {% trans "Add group" as add_group_str %}
-    {% include "wagtailadmin/shared/header.html" with title=add_group_str icon="group" %}
+    {% include "wagtailadmin/shared/header.html" with title=add_group_str icon="group" only %}
 
     <div class="nice-padding">
         <form action="{% url 'wagtailusers_groups:add' %}" method="POST" novalidate>