Fix custom withdrawal fee bug

[ghost]

The wallet code was by default using the value on Setting/Preferences instead of the value chosen on the Withdrawal screen.

Fixes #5575

• Pass the chosen fee rate to the wallet when doing a withdraw.
• Calculate sats/vb in the summary popup instead of showing the value of the input text field.

Steps to reproduce the issue (regtest)

• Prerequisite: Use custom withdrawal fee is unchecked in Settings/Preferences.
• Go to Funds/Receive funds select a receiving address and copy to clipboard.
• Go to Funds/Send funds, paste your address into the withdraw to address field.
• Take note of the default suggested transaction fee (in this case 50 sats/vb).
• Click use custom value for fee, enter 2 sats/vbyte or the lowest value permissible.
• Click Withdraw Selected. Note the mining fee and the sats/vb reported alongside it.
• Click yes to withdraw.
• Now go to the log where it details the transaction, note the weight e.g. 110 virtual bytes, and note the fee paid e.g. 0.00055 BTC ( = 55000 sats).
• Divide the fee paid by virtual bytes to get the actual fee per vb paid, in this case 55000 / 110 = 50 sats/vb.
• The fee used is the value originally shown in the fee text box, instead of the one the user entered.

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[update] Refactored the fix to pass the chosen fee rate to the wallet instead of setting the custom withdrawal flag in user preferences. I think this approach is cleaner and more in line with @BtcContributor original idea.

Also: fixes #5605

[ripcurlx]

utACK

[MatthewCroughan]

#5601