Move transaction callbacks config into initializer

Stems from comment by @dhh: https://github.com/rails/rails/pull/17227#commitcomment-9079727:

> I'd like for the option to be put in an initializer instead, though.
> It's not worthy of such high promise as the application.rb.

Style-wise, I am always adding the new initializer file to a Rails app, even
when the app is created with `--skip-active-record`. In this case, the line of
code in the initializer is commented out.

I think this is better than **skipping** the initializer and is more coherent
with the other initializers. Is that okay? :christmas_tree: