Add email logging feature

Email can be complicated, with lots of potential issues related to configuration, deliverability, etc.

To help with auditing emails that a site sends, including debugging and fixing any issues, we need a good email logging system. I think this is valuable enough for our target market to be included with ClassicPress, probably as a core plugin.

I'm imagining basically a custom post type with a lightly customized archive page, like a few meta fields about the email and whether it was sent successfully or not. This could also include an option to resend previous emails and an option to disable email sending entirely for local development, except for logging the emails that would have been sent.

https://wordpress.org/plugins/wp-mail-logging/ already does a lot of this.

Semi-related discussion and links to other related petitions here: https://github.com/ClassicPress/ClassicPress/issues/363

In particular https://petitions.classicpress.net/posts/163/smtp-integration-settings-as-core-plugin would be a fairly natural tie-in here. It would be great if the SMTP settings used to send each email would also be saved as part of the archive metadata.