Raise early when Active Storage is not setup (Shopify#392)

Instead of waiting until a CSV file is uploaded, this raises an
exception early when a CSV Task is defined by calling csv_collection and
Active Storage is not loaded or its tables created.

I reused part of the message that Rails emits when a file is attached
and one of the Active Storage tables does not exist.

app/tasks/maintenance_tasks/task.rb

@@ -38,6 +38,10 @@ def available_tasks
       # An input to upload a CSV will be added in the form to start a Run. The
       # collection and count method are implemented.
       def csv_collection
+        if !defined?(ActiveStorage) || !ActiveStorage::Attachment.table_exists?
+          raise NotImplementedError, "Active Storage needs to be installed\n"\
+            "To resolve this issue run: bin/rails active_storage:install"
+        end
         include(CsvCollection)
       end