Ensure activity plugin is loaded before migrating old revisions

Otherwise you get an `package_activity_list` action not found error

ckan/migration/migrate_package_activity.py

@@ -32,6 +32,7 @@
 import argparse
 from collections import defaultdict
 from typing import Any
+import sys
 
 
 # not importing anything from ckan until after the arg parsing, to fail on bad
@@ -274,6 +275,8 @@ def print_errors(errors):
     args = parser.parse_args()
     assert args.config, u'You must supply a --config'
     print(u'Loading config')
+
+    from ckan.plugins import plugin_loaded
     try:
         from ckan.cli import load_config
         from ckan.config.middleware import make_app
@@ -291,6 +294,11 @@ class Options(object):
             cmd._load_config()
             return
         load_config(args.config)
+
+    if not plugin_loaded("activity"):
+        print("Please add the `activity` plugin to your `ckan.plugins` setting")
+        sys.exit(1)
+
     if not args.dataset:
         migrate_all_datasets()
         wipe_activity_detail(delete_activity_detail=args.delete)