Fs image links cleanup

This is from the discussion with @chris-allan https://github.com/openmicroscopy/openmicroscopy/pull/759. Remove unnecessary methods from Blitz Gateway and limit the exposure of Fileset / FS specific code in web templates.

Shouldn't change behaviour atall. 

NB: This creates a 'breaking change' in the Blitz Gateway API since getArchivedFiles() is replaced with getImportedImageFiles(). We could keep getArchivedFiles() - Just not so clear that this includes Filesets.