urlpatterns: Avoid warning for name with colon

Rename "wagtailadmin_pages:add_subpage" to
"wagtailadmin_pages_add_subpage" to avoid the warning

    ?: (urls.W003) Your URL pattern '^admin/pages/(\d+)/add_subpage/$' [name='wagtailadmin_pages:add_subpage'] has a name including a ':'. Remove the colon, to avoid ambiguous namespace references.

config/urls.py

@@ -31,7 +31,7 @@
     path('admin/users/', include('mpicms.users.urls')),
 
     # Override wagtail add_subpage view
-    re_path(r'^admin/pages/(\d+)/add_subpage/$', add_subpage, name='wagtailadmin_pages:add_subpage'),
+    re_path(r'^admin/pages/(\d+)/add_subpage/$', add_subpage, name='wagtailadmin_pages_add_subpage'),
 
     # Add subscribe views
     re_path(r'^admin/pages/(\d+)/subscribe/$', subscribe, name='wagtailadmin_pages_subscribe'),