Remove redundant `|safe` filter

All button classes defined in wagtail.admin.widgets.button use either format_html or render_to_string, which mark their results as safe. And if you pass some other object as part of the buttons list that isn't as well-behaved, it's probably about time you fixed that...

wagtail/admin/templates/wagtailadmin/pages/listing/_buttons.html

@@ -1,5 +1,5 @@
 {% for button in buttons %}
     {% if button.show %}
-        <li>{{ button|safe }}</li>
+        <li>{{ button }}</li>
     {% endif %}
 {% endfor %}

wagtail/snippets/templates/wagtailsnippets/snippets/listing_buttons.html

@@ -1,5 +1,5 @@
 {% for button in buttons %}
     {% if button.show %}
-        <li>{{ button|safe }}</li>
+        <li>{{ button }}</li>
     {% endif %}
 {% endfor %}