Disable swap button if Alice sells all her bitcoin

[luckysori]

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When selling bitcoin, Alice must add some more on top of the sell amount selected to cover the transaction fee. This is not possible if her sell amount already uses up all of her bitcoin funds.

We currently only disable the swap button if the sell amount goes over her bitcoin amount, but we could account for the transaction fee and disable the button a little earlier, to prevent an error from occurring when trying to collect her UTXOs for the swap.

Originally posted by @bonomat in https://github.com/comit-network/waves/pull/136#discussion_r576457883