Draft: Extension to create shared thumbnail repositories

I'm developing this using the existing terminal extension because for some reason my IDE is disabled if I try working on a new extension (even though it compiles and works). When it is ready I will, of course, create a separate extension.

To create the shared thumbnail repositories it just takes the existing thumbnails and puts them in the correct place. Right now the main issue is that it isn't guaranteed that all the required thumbnails exist.