fix(smart-contracts): refactor contract to address pfc comments

[nseguias]

This PR does the following:

1. Moves the start bid process from Instantiate to a separate function to be reused on SettleAuction and not repeat code.
2. Factory subdenom has now a more meaningful name auction.<counter> to have some context when checking wallet balances.
3. Modifies the way the contract owner is updated, using cw_ownable.
4. Changes the way it adds or removes whitelisted addresses to prevent overriding the list of whitelisted addresses by mistake.
5. ExitPool now checks if the contract has the funds locked and only allows people exiting the pool of they are not. Funds are locked on the first bid and unlocked when settling the auction.
6. Bidding balance tracks the amount of native funds that can be used for bidding, and makes sure there are always funds available for creating a new denom.
7. Instantiate now needs to send some native tokens that will stay in the contract for creating new denoms. The minimum balance is then saved into Config

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