Add MMS support to SMS Advanced Alerts

[jdrummey]

There are two ways to send an SMS message using VizAlerts:

1. Use the email-to-SMS gateways that some mobile providers have, for example [email protected] in the USA. AT&T also has an email-to-MMS gateway at [email protected] where inline images in emails are automatically converted for MMS.

2. Use an Advanced Alert with an SMS action. This enhancement is about adding MMS support to the SMS Advanced Alerts. This is pretty straightforward for the Twilio integration, any VIZ_IMAGE() references in an SMS alert would be attached to the message object.