Amount validation in Send form is not taking into account the fee

[johnny9]

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Note that when entering full balance, Review is highlighted but not clickable.

As the GUI won't allow creating tx's with 0 fees, maybe Review should only be highlighted when amount < balance

Originally posted by @MarnixCroes in #462 (review)

This requires dynamic fee estimation to be added to the Send form in order to calculate.

[johnny9]

Also in #490