This commit changes the WC action that MailPoet uses to hook into WC and
customize the footer and the header of WC e-mails. Before, MailPoet was
using the woommerce_init action. The problem with using this action to customize
the e-mails is that it runs on every request. Furthermore, since we were calling
WC->mailer() inside the callback, we were instantiating WC_Emails on
every single request (WooCommerce instantiate this class only when
needed).
Replacing woocommerce_init with woocommerce_email means that our code
will run only when needed (right after WooCommerce adds callbacks to the
actions woocommerce_email_header and woocommerce_email_footer) and that
we won't be unnecessarily instantiating WC_Emails ourselves. We get the
singleton instance of this class as a parameter.
[MAILPOET-3421]
