Handle image annotations on activity pages

We're not going to be able to show image thumbnails on activity pages, but one way or another image annotations need to be rendered somehow. Options:

1. Don't do anything, and just let image annotations look the same as page notes
1. Just indicate that the annotation is an image annotation and what page of the PDF it's one
2. Possibly add a visualization of the relative location of the image annotation on the page

Start by locating the activity-page rendering entry point and the code that displays image annotations. Review the three proposed behaviors and determine which one is accepted; the work is done when the chosen image-annotation treatment is rendered on activity pages and its behavior is verified.

Written by the indexing model from the issue text.