R1:cfcd35d8a390

Remove standalone SMS support in favor of a "Mail, SMS, and other media are mostly the same thing" approach

Summary:
Ref T920. Over time, mail has become much more complex and I think considering "mail", "sms", "postcards", "whatsapp", etc., to be mostly-the-same is now a more promising avenue than building separate stacks for each one.

Throw away all the standalone SMS code, including the Twilio config options. I have a separate diff that adds Twilio as a mail adapter and functions correctly, but it…