Improve onboarding type detection on oauth and subscriptions

The following discussions from !126567 (merged) should be addressed:

• @dstull started a discussion:

  note (non-blocking): resolve oauth comparison

  base_stored_user_location_path.starts_with?(::Gitlab::Routing.url_helpers.oauth_authorization_path) could probably just be

  base_stored_user_location_path == ::Gitlab::Routing.url_helpers.oauth_authorization_path

• @dstull started a discussion:

  note (non-blocking):

  subscription is detected by seeing stored redirect value... I wonder if we can just do this by passing a param of subscription instead. This would match trial param use.