Media viewer thumbnails don't take viewport width into account

[ybnd]

Describe the bug When running with mediaViewer.image = true, thumbnails on Item pages have a fixed width which doesn't look right on screen widths between 768px and 1200px

To Reproduce Open any Item on the demo server (or start a local instance with mediaViewer.image = true) Resize the screen somewhere between 768px and 1200px

Expected behavior The "Media viewer thumbnail" should resize to fit in the left column without overlapping with the right column

Related work Could be fixed at the same time as https://github.com/DSpace/dspace-angular/issues/1887

[tdonohue]

@ybnd : It looks like you claimed the related ticket. I'll assign this to you as well & pull these over to the 7.5 board so they can be worked on when you are ready.