Make the djangocms_blog column rename conditional

Databases created from scratch already have customuser_id, so the
unconditional rename failed on them.

membership/migrations/0008_change_user_id_to_customer_id_in_djangocms_blog.py

@@ -1,5 +1,23 @@
 from django.db import migrations
 
+# Only rename when the old column exists: databases created from scratch
+# already have customuser_id (djangocms_blog is created after AUTH_USER_MODEL
+# is set), so an unconditional rename fails there.
+RENAME_IF_NEEDED = """
+DO $$
+BEGIN
+    IF EXISTS (
+        SELECT 1 FROM information_schema.columns
+        WHERE table_name = 'djangocms_blog_authorentriesplugin_authors'
+        AND column_name = 'user_id'
+    ) THEN
+        ALTER TABLE djangocms_blog_authorentriesplugin_authors
+        RENAME COLUMN user_id TO customuser_id;
+    END IF;
+END
+$$;
+"""
+
 
 class Migration(migrations.Migration):
     dependencies = [('membership', '0007_auto_20180213_0128'),
@@ -7,7 +25,5 @@ class Migration(migrations.Migration):
                     ]
 
     operations = [
-        migrations.RunSQL(
-            "ALTER TABLE djangocms_blog_authorentriesplugin_authors "
-            "RENAME COLUMN user_id TO customuser_id;"),
+        migrations.RunSQL(RENAME_IF_NEEDED),
     ]