Describe the bug
ds-media-viewer was recently enabled on the demo server, but has some issues right now

• Restricted thumbnails or image content are not shown. This was recently addressed for ds-thumbnail in Support restricted thumbnails #1694.
  Since we're using ngx-gallery for this it's not immediately clear how to implement the same "retry with token" approach though.
  

• When used for regular thumbnails, images are centered in the left column
  

The original code for these components is a bit messy, so I'd suggest we also refactor it as we address these issues

• Nested subscriptions
• Assumes every ORIGINAL Bitstream has a thumbnail
• Assumes that Bitstreams in the ORIGINAL & THUMBNAIL Bundles are in the same order, which may not be the case → AFAIK the right way would be to match names?

To Reproduce
Steps to reproduce the behavior:

1. Run DSpace Anguar with mediaViewer.image: true
2. Open an Item with a thumbnail
   • The thumbnail will be centered relative to the left column
3. Find (or create) an Item with a thumbnail that only admin users can read
   • For admin users, the thumbnail will not show up on the Item page (but will show up on e.g. the search page)
   • For anonymous or non-admin users, the thumbnail will not show up

Expected behavior
ds-media-viewer should work with restricted content and look similar to ds-thumbnail when used for thumbnails.
When restricted content cannot be retrieved the placeholder should be shown.

Related work
#1694