Email: Allow users to add images to emails

[dospromptman]

Currently, images are not supported within the body of emails sent using the send emails action.

We should allow the user to add images using the rich text editor in the send emails action.

Note that a decision will need to be made about how the images are included in the email: either as inline images as part of a multi-part message body, or as URL references. Both ways introduce significant complication in how the images are added/supported using the rich text editor, size limits (and how those limits can be enforced) for inline images, and sharing settings (content delivery or asset conversion) for images that are included as URL references. A concrete strategy (or a reasonable plan for how these settings can be controlled by the user) for how images should be allowed to be added and included in emails should be determined before starting work on this feature.