Add campaign UTM links and piha.li shortener as a core helper.

Keep mint/UTM in always-on core so email_sms and directmail stay optional, and brand utm_source from SITE_NAME (or UTM_SOURCE) instead of a hardcoded client name.
Closes westfarn/web_django_template#3

site/directmail/urls.py

@@ -13,6 +13,11 @@ urlpatterns = [
         name="campaign_status_json",
     ),
     path("campaigns/<uuid:pk>/send/", views.campaign_send, name="campaign_send"),
+    path(
+        "campaigns/upload-image/",
+        views.campaign_image_upload,
+        name="campaign_image_upload",
+    ),
     path(
         "campaigns/<uuid:pk>/messages/<uuid:message_id>/remove/",
         views.campaign_message_remove,