Mailer should support plaintext fallback

[alexweissman]

We should take advantage of PHPMailer's AltBody option to set plaintext fallbacks for HTML email messages. This would help with mail client compatibility, and also help improve SpamAssassin scores. Here is an example of a SpamAssassin report I received after sending a message with UserFrosting:

SpamAssassin Score: 1.851
Message is NOT marked as spam
Points breakdown: 
-0.0 SPF_HELO_PASS          SPF: HELO matches SPF record
 0.0 HTML_MESSAGE           BODY: HTML included in message
 1.1 MIME_HTML_ONLY         BODY: Message only has text/html MIME parts
 0.1 DKIM_SIGNED            Message has a DKIM or DK signature, not necessarily valid
 0.0 T_DKIM_INVALID         DKIM-Signature header exists but is not valid
 0.6 HTML_MIME_NO_HTML_TAG  HTML-only message, but there is no HTML tag

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[alexweissman]

Still an issue...