Allow adding some custom context to the rendered spark mails preview

spark/spark_mails/admin.py

@@ -1,10 +1,21 @@
+from django.conf import settings
 from django.contrib import admin
 from django.utils.html import format_html
+from django.utils.module_loading import import_string
 from django.utils.translation import ugettext_lazy as _
 
+from django.utils.translation import ugettext_lazy as _
 from .models import Mail
 
 
+def spark_mails_context(instance):
+    return {}
+
+
+if getattr(settings, "SPARK_MAILS_CONTEXT", None):
+    spark_mails_context = import_string(settings.SPARK_MAILS_CONTEXT)
+
+
 @admin.register(Mail)
 class MailAdmin(admin.ModelAdmin):
     list_display = ["event_group"]
@@ -18,7 +29,7 @@ def rendered(self, instance):
             return format_html(
                 '<div style="white-space:pre-wrap; max-width:40rem">'
                 "<code>{}\n\n{}</code></div>",
-                *instance.render({}),
+                *instance.render(spark_mails_context(instance)),
             )
         except Exception:
             return format_html('<div style="color:red">INVALID</div>')