fix #10175, #10953: shorten image name

Fixes http://trac.openmicroscopy.org.uk/ome/ticket/10175. To test, try importing images from absurdly long-named/deep paths. Cc: @manics.

--no-rebase as it's FS-specific.

Update: Additional test: Try importing images and check that the image name (also in DB as `image.name`) fixes https://trac.openmicroscopy.org.uk/ome/ticket/10953 but `filesetentry.clientpath` remains the full client-side path.