MediaGallery: fall back on `url` when `preview_url` isn't provided

app/soapbox/components/media_gallery.js

@@ -168,7 +168,7 @@ class Item extends React.PureComponent {
           onClick={this.handleClick}
           target='_blank'
         >
-          <StillImage src={previewUrl} alt={attachment.get('description')} />
+          <StillImage src={previewUrl || originalUrl} alt={attachment.get('description')} />
         </a>
       );
     } else if (attachment.get('type') === 'gifv') {

app/soapbox/features/account_gallery/components/media_item.js

@@ -88,7 +88,7 @@ class MediaItem extends ImmutablePureComponent {
 
       thumbnail = (
         <StillImage
-          src={attachment.get('preview_url')}
+          src={attachment.get('preview_url') || attachment.get('url')}
           alt={attachment.get('description')}
           style={{ objectPosition: `${x}% ${y}%` }}
         />