Not possible to enable video viewer without also enabling image viewer

[tdonohue]

Describe the bug
Currently, it's not possible to enable the (out-of-the-box) video viewer without also enabling the image viewer. So, a configuration like this is not valid:

mediaViewer:
  image: false
  video: true

Instead, to enable the video viewer you MUST configure it like this:

mediaViewer:
  image: true
  video: true

This appears to be because of the design of the media-viewer.component. When that component is used, it always enables the image viewer, but only enables the video viewer if video=true. See the display logic in https://github.com/DSpace/dspace-angular/blob/main/src/app/item-page/media-viewer/media-viewer.component.html

To Reproduce
Steps to reproduce the behavior:

1. Attempt to only enable the video viewer:

mediaViewer:
  image: false
  video: true

2. Notice that it won't work unless you also set image: true

Expected behavior
It should be possible to enable the video viewer only. This can be useful if an institution wishes to use a different viewing mechanism for images (e.g. enabling IIIF integration instead)