Add support for lazy strings in Draftail widget options

Ensure that the JSON encoder can correctly handle lazy strings (such as reverse_lazy and gettext_lazy) so that there is better support for admin URLs and translations that may not be able to resolved when the application starts.

wagtail/admin/rich_text/editors/draftail/__init__.py

@@ -2,7 +2,9 @@ import json
 import warnings
 
 from django.forms import Media, widgets
+from django.urls import reverse_lazy
 from django.utils.functional import cached_property
+from django.utils.translation import gettext_lazy
 
 from wagtail.admin.rich_text.converters.contentstate import ContentstateConverter
 from wagtail.admin.staticfiles import versioned_static
@@ -11,6 +13,21 @@ from wagtail.telepath import register
 from wagtail.widget_adapters import WidgetAdapter
 
 
+class LazyStringEncoder(json.JSONEncoder):
+    """
+    Add support for lazy strings to the JSON encoder so that URLs and
+    translations can be resolved when rendering the widget only.
+    """
+
+    lazy_string_types = [type(reverse_lazy("")), type(gettext_lazy(""))]
+
+    def default(self, obj):
+        if type(obj) in self.lazy_string_types:
+            return str(obj)
+
+        return json.JSONEncoder.default(self, obj)
+
+
 class DraftailRichTextArea(widgets.HiddenInput):
     template_name = "wagtailadmin/widgets/draftail_rich_text_area.html"
     is_hidden = False
@@ -65,7 +82,9 @@ class DraftailRichTextArea(widgets.HiddenInput):
 
     def get_context(self, name, value, attrs):
         context = super().get_context(name, value, attrs)
-        context["widget"]["options_json"] = json.dumps(self.options)
+        context["widget"]["options_json"] = json.dumps(
+            self.options, cls=LazyStringEncoder
+        )
         return context
 
     def value_from_datadict(self, data, files, name):