* Prevent different identities from a same SSO provider from accessing a same account
* Lock auth provider changes behind `ALLOW_UNSAFE_AUTH_PROVIDER_REATTACH=true`
* Rename methods to avoid confusion between OAuth and OmniAuth
* fix: change Identity's id column to a bigint
This appears to be the last model created using a 5.0 migration,
where column types defaulted to `integer` rather than `bigint`.
This migration changes the column type to match that of all of the
other ID columns.
* Change user_id column in identities to bigint and fix down-migration
