Handle legacy file attachments that lack a file_hash

I am sure that there is a better way to do this, but I was in a hurry. I am submitting it, so at least you have the option of using it.

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SMF forums originating from past versions can have a large number of legacy attachments that lack a file_hash. Currently, the importer encodes these legacy files with a new file_hash, copies the attachment over into ElkArte, but then orphans them by not saving the new file_hash in the ElkArte database.

This update simply identifies the legacy files that lack a file_hash, then imports them over into ElkArte using a static 'legacy' file hash. This is a bit of a kludge, but it seems better than the current system that orphans the legacy attachments.

Signed-off-by: Noteworthy Software, Inc <online@noteworthysoftware.com>