feat: separation - endpoints for finalize & assign

finalize at:
    POST /v1.0/vaults/{vaultid}/files/{fileid}
assign at:
    POST /v1.0/vaults/{vaultid}/files/{fileid}/blocks
Wait, we are manually doing this?  Then we should change it all.  Can someone create a card for it?

There is only one other place https://github.com/rackerlabs/deuce/blob/73188b1841f4f80f32b9ca30d998e5c86eb4d19b/deuce/transport/wsgi/v1_0/files.py#L122