User activity tracking

If you just use `change` when the migration involves adding columns.  A migration is only irreversible if information about the previous state of your schema is lost during the migration.  In this case, all you need to know in order to _remove_ a column is the table and column involved, both of which are given to `add_column`.

Conversely, if your "up" migration _removed_ a field with something like

```ruby
remove_column :user, :obsolete_field
```

there's no way to "reverse" this migration because we have no idea what type of field `obsolete_field` was, whether it had a default value, whether it could be `NULL`, etc.