Bump vue-loader from 15.5.1 to 15.6.0 in /apps/oauth2

Bumps [vue-loader](https://github.com/vuejs/vue-loader) from 15.5.1 to 15.6.0.
- [Release notes](https://github.com/vuejs/vue-loader/releases)
- [Changelog](https://github.com/vuejs/vue-loader/blob/master/CHANGELOG.md)
- [Commits](https://github.com/vuejs/vue-loader/compare/v15.5.1...v15.6.0

)

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