Ticket435

Image plugin changes for ticket 435.

1. Use icon from mockup and adjust spacing between icon and "Describe" 
* to [triangle icon] "*Describe the image for someone who cannot see it.* This description can be read aloud, making it possible for visually impaired learners to understand the content."

4. If the author does not provide a caption, when the image is placed in the document, see the mockup for the behavior of "Description missing".
* use [triangle icon] from the mockup,_ italics_ gray text "_Description missing_"