Mailchimp for WordPress Premium 4.9.3

- E-commerce: use `DateTime::ATOM` as ISO8601 datetime format to prevent an issue with order dates on some PHP versions.

- Forms: Add defer attribute to AJAX forms script.
- Add Mailchimp Activity feature plugin.
- Updated compatibility with Mailchimp for WordPress v4.9.0.

- Forms: Minor improvements to autocomplete feature for improved performance & decreased memory usage.
- E-commerce: Bail early if guest cart has no valid email address yet.
- E-commerce: Shrink size of cart tracking script by 30%.
- In general, more minor performance, memory use and size optimizations for all bundled JavaScript code.