Add a deposit when creating a supersig

[gdnathan]

I would say that at multisig creation the creator transfer an amount equal to PreimageByteDeposit (that we rename) * size of the stored data (Supersig + Supersig.members) to the multisig account. This amount is reserved (same way that for PreimageCall).
And when the delete_multisig is called, it unreserve those funds and transfer them to an account passed as argument in the call. It also decrement the account reference counters, and remove the superSig from storage. This call being only callable by the superSig itself.

From a high level users can delete their multisig when not used anymore, and are incentivized to do so by this reserved amount

Originally posted by @tdelabro in #2 (comment)

[gdnathan]

fixed