consent signature image, sent via email

This change involved re-writing SendEmailViaAmazonService to use the sendRawEmail API. This allows us to attach the signature image as an inline attachment. This change also involves updating the consent templates to use inline attachments.

This change was unit tested and was manually tested on Gmail and Yahoo Mail, on desktop and on an Android device. This change was also tested with various members of bridge@sagebase.org, including one confirmed iOS.