Fs image links

This branch introduces links between filesets and images/plates as the primarily output of any import. These links are a prerequisite for being able to properly cleanup any files on an image or plate delete. They also enable returning more than just the Pixels to the client once the importMetadata step is finished, but this will need to be further integrated into insight (cc @jburel & @scottlittlewood)

For testing, the primary client-visible change is that deleting an image or a plate (in the regular way) should remove all the files under the ManagedRepository directory, though intermediate directories will be left dangling.