Teach the API to send emails

This patch makes it so that the API will send emails when a talk comment is created.  Things to note:
 - ~~this assumes we have composer on live; we don't at this point~~ update: composer dependency removed
 - this requires config changes for everyone, pull the email section from config.php.dist into config.php
 - the above config change is required for live as well so check before merging
 - there is a PR on the VM repo for adding mailcatcher to the VM, you might find this helpful to test this patch
 - at least half the credit goes to @Tocacar :)

The patch supports (or should support) multiple speakers, plain text as well as HTML emails, the ability to override the TO address to send all emails to a specific address (also useful for testing) and a configurable from address.