[Cashtab] Migrate validateMnemonic from useWallet to validation

Summary:
T2800

This diff:

• moves the validateMnemonic function from useWallet to validation
• adds corresponding unit tests to validate.test.js
• updates references to validateMnemonic in the Onboarding and Configure components

Test Plan:

• grep -r 'validateMnemonic' src/ and ensure no references to useWallet's ContextValue object
• npm test
• npm start
• create a new wallet, note the mnemonic, delete wallet
• import the wallet via mnemonic
• ensure successful send and receive of XEC to this imported wallet
• open browser in incognito mode to trigger onboarding screen
• create a new wallet and ensure successful creation
• import the earlier wallet via mnemonic
• ensure successful send and receive of XEC to this imported wallet

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12586