[POS-28] Remove ftsSeedLength, improve secretToFtsSeed
[POS-28] Remove ftsSeedLength, improve secretToFtsSeed

input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl