Fix add flag link in Wagtail 2.10.

Wagtail 2.10 changes the "add_*" variables for the button link that appears in the top right of the header to "action_*". This change supports that while remaining backwards compatible with "add_*".

wagtailflags/templates/wagtailflags/includes/header.html

@@ -4,6 +4,10 @@
   <li>{{ title }}</li>
 </ul>
 {% include "wagtailadmin/shared/header.html" with title=title icon=icon %}
+{% elif wagtail_header_action %}
+{# Wagtail 2.10 changes "add_*" in the shared admin header to "action_*" #}
+{% url 'wagtailflags:create_flag' as add_link %}
+{% include "wagtailadmin/shared/header.html" with title=title action_icon=icon action_text='Add flag' action_url=add_link %}
 {% else %}
 {% include "wagtailadmin/shared/header.html" with title=title icon=icon add_text='Add flag' add_link='wagtailflags:create_flag' %}
 {% endif %}

wagtailflags/views.py

@@ -1,6 +1,8 @@
 from django.http import Http404
 from django.shortcuts import get_object_or_404, redirect, render
 
+import wagtail
+
 from flags.models import FlagState
 from flags.sources import get_flags
 from flags.templatetags.flags_debug import bool_enabled
@@ -11,6 +13,8 @@
 def index(request):
     context = {
         "flags": sorted(get_flags().values(), key=lambda x: x.name),
+        # Wagtail 2.10 changes "add_*" in the shared admin header to "action_*"
+        "wagtail_header_action": wagtail.VERSION >= (2, 10, 0),
     }
     return render(request, "wagtailflags/index.html", context)