Use mime attachements to embed avatars and other icons into html emails

This will mean avatars/icons display in mail clients where in situations that they can't resolve the img hrefs

• when external image loading is disabled for security
• when the href points to somewhere (DMZ) that the mail client can't see
• when the mail client is considered the anon user by the jira serving the image (they get the bog-standard avatar, not the customized avatar)