Update the SMTP (PHPMailer) adapter for the new mail API; remove "encoding" and "mailer"

Summary:
Ref T920. Ref T12404.

• Update to the new "$message" API.
• Remove "encoding". I believe "base64" is always the best value for this since we stopped seeing issues once we changed the default.
• Remove "mailer". This is a legacy option that makes little sense given how configuration now works.
• Rename to "SMTP". This doesn't affect users anymore since this mailer has been configured as smtp for about a year.
• This does NOT add a timeout since the SMTP code is inside PHPMailer (see T12404).

Test Plan: Sent messages with many mail features via GMail SMTP and SendGrid SMTP.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12404, T920

Differential Revision: https://secure.phabricator.com/D19961