Show sources on the media asset show page. An asset can have more than one source if the same
file is uploaded from multiple sites.
Only sources from known sites are shown. Sources from unknown sites aren't shown because they
could potentially contain private information or identify the uploader in some way.
Known issue: Twitter posts often show two sources, the direct image URL and the page URL. This is
because someone uploaded the direct image URL first, and we're not able to tell that the image URL
and the page URL are for the same tweet.
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