Make NO ACTION the default action for foreign keys

[samtay]

See #1416 for motivation, but tl;dr is that NO ACTION is a more sensible default across the supported databases.

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[parsonsmatt]

Sorry I've taken so long to get to this!

I'm amenable to the rationale for the changes in this, however, I'd like to think through the upgrade path. If we release this as-is, then every persistent user is going to get a new migration suggestion to change their existing RESTRICT foreign key changes to NO ACTION, for every foreign key. While this may be preferable, it's also a pretty harsh UX. We'd essentially be requiring all users to modify all foreign key relationships and specify Restrict instead of using that as an implied default.

Instead, it'd be better to do this in phases:

1. Introduce the NoAction constructor and type, but Restrict is still default.
2. Allow the end user to customize the default in settings somewhere. Instead of using Restrict as a hardcoded default, we'll refer to the relevant settings record.
3. Switch the default to NoAction and publicize the change.