Issue Summary

Post mortem images have a gray preview placeholder whenever added, but the previews never finish loading. 

Steps to Reproduce

1. Start a Postmortem for an incident
2. In the editor, select the "Image" tool
3. Paste a valid URL for an image file

Expected Results

Able to see a preview of the image in the editor

Actual Results

A grey placeholder appears, but the image itself never loads

Workaround

As long as the image URL is valid, publishing the Postmortem will still pull in the file.