Use Terry's LITImageMagick PDF thumbnail media filter

We had one-off enabled the GWU LIT ImageMagick media filter for some of our clients
https://github.com/Georgetown-University-Libraries/DSpaceImageMagickThumnails

But with its formal contribution to DSpace/DSpace, we can just merge in this mainstream code into our DSpace, to simplify deployment.

See the Pull Request where this is being discussed:
https://github.com/DSpace/DSpace/pull/597