Fix checking the maximum amount user can send

[bartekn]

https://github.com/stellar/account-viewer/blob/master/controllers/send-widget.controller.es6

        let minimumBalance = 20 + (account.subentry_count) * 10;
        let nativeBalance = _(account.balances).find(balance => balance.asset_type === 'native').balance;
        let maxSend = new BigNumber(nativeBalance).minus(minimumBalance);
        if (maxSend.lt(this.amount)) {
          throw new BasicClientError('InsufficientBalanceError', {maxSend});
        }

We're not subtracting transaction fee from maxSend. Thanks @briangale for catching this!

[AmodD]

A simple fix for subtracting Txn fee would be to hard code .minus(0.00001) assuming single operation.

But an improvement in the code should involve fetching base fee.

single send operation

Base Fee hard coded in stellar-base_transactionBuilder class