Implement Personal Data Export for author information

[moorscode]

WordPress 4.9.6 introduces new tools to ease GDPR required data exports.

As we also store data that belongs to a user, we want to hook into these tools and provide the data for exports.

Data:

• User meta: wpseo_title
• User meta: wpseo_metadesc

Reference: https://developer.wordpress.org/plugins/privacy/adding-the-personal-data-exporter-to-your-plugin/

As these filters only are used when the specific WordPress version is used, we don't have to add specific version checks for this.