Replacing an image in the Media Library breaks joins to that image

To reproduce:

- Login to the http://demo.apostrophecms.org
- Join an image to a User (Info tab)
- Replace that image in the media library
- Edit the user in step 2 and view their info tab (404 getting their image)

Start by reproducing the issue in the demo site: join an image to a User, replace that image in the Media Library, and reopen the user's Info tab. Trace the media replacement and image-join handling; done means the existing User join still resolves the replaced image instead of returning a 404.

Written by the indexing model from the issue text.