Validate text limit implemented in oauth_applications.scopes

Follow-up of #515871 (closed)

This issue tracks the migration series from #515871 (closed) and completes the oauth_applications.scopes field migration by validating the text limit constraint.

In particular, the issue is completing the following step: https://docs.gitlab.com/development/database/strings_and_the_text_data_type/#validate-the-text-limit-next-release