REL-1394: Now suppressing e-mail notifications when NGO flags are unchec...

...ked when auxiliary files are uploaded as requested.
Instead of an explicit `suppressNotifications` flag and explicitly marking files as unchecked, what if the `BackendFileSystemImpl` just handled that as part of storing the file?  You could see it as part of establishing the metadata for files when they are accepted.  So when the last chunk comes in it could set the checked flag to `false`.  It won't cause any notifications because it happens underneath the `NotifyingBackend`.

(Also, a trivial detail that would go away anyway if this suggestion makes sense is to remember to mark everything `final`.)