Improve Mnemonic + Password handling security

[JSKitty]

Abstract

Previously, MPW left 'residue' of sensitive data such as Passwords and Mnemonics in memory (by keeping it in the HTML input values), thus, for example, after creating a new MPW wallet - as long as that initial session is open, your mnemonic is recoverable. The same goes for passwords; as long as the Prompt box was not reused, they were recoverable.

This PR fixes this by immediately wiping sensitive data from these input boxes as soon as possible.

Testing

Try this console command after Creating a new wallet:
document.getElementById('ModalMnemonicContent').innerText

Try this console command after Unlocking your wallet:
document.getElementById('restoreWalletPassword').value

In master MPW, those will return sensitive data - in this PR, all sensitive input boxes should be cleanly empty immediately after completing an action (Unlock, New Wallet, etc).

What does this PR address?

It addresses a potential, and very simple security leak in the UI, which could've been exploited by malware or even a malicious user with access to one's device locally or remotely.

What features or improvements were added?

N/A, security improvement only.

How does this benefit users?

MPW is slightly more resilient to easy attacks - yay!

[Liquid369]

tACK f20be50

Nice catch especially before 1.0

[Duddino]

tACK