Upgrade path from references module (node_reference and user_reference)

[jenlampton]

1. We should provide an automatic upgrade of references fields when upgrading from D7 to backdrop, since the reference module is the direct successor of that project.

2. For existing Backdrop sites running references module, we'll need to provide them with an opt-in to convert their references fields to reference fields.

@moonray started work on an update function in pull request #25, but we'll separate out all the upgrades into separate issues so they can get merged in as they are ready.

[jenlampton]

I've pushed up a new PR that will upgrade a Drupal 7 site using references to reference automatically, but if you are using a Backdrop site with references you need to opt-in to the conversion by adding $settings['reference_convert_references'] = TRUE; to your settings.php file.

I don't think the settings.php value is the best place for the opt-in long-term, but it was the fastest thing to do to get a PR up that would do the trick :)

It might be better to also have a UI, so that people who don't have access to settings.php (like in production) would still have a way to opt-in and convert their fields.