[WP] Support HTML formatted emails in core

This was an idea from @herbdool in this comment but I didn't want it to get lost, so creating a separate issue to track.

WordPress provides HTML formatted emails, and it may be something our target audience really needs help with. Maybe we should look at what WP does and see if there's something simple we could do that's similar?

For reference: https://wordpress.stackexchange.com/questions/27856/is-there-a-way-to-se...