[Cashtab] [BCH Deprecation] [Mnemonic] Pt 5/7 - Deprecate BCH.Mnemonic

Summary:
T2730

Depends on D12466

Stacked diff to localize existing uses of BCH.Mnemonic throughout the app and deprecate the BCH.Mnemonic object.

Pt 1/7 - Implement BCH.Mnemonic.toSeed locally
Pt 2/7 - Implement BCH.Mnemonic.generate locally
Pt 3/7 - Implement BCH.Mnemonic.wordLists locally
Pt 4/7 - Implement BCH.Mnemonic.validate locally
Pt 5/7 - Deprecate BCH.Mnemonic
Pt 6/7 - Optimize bip39 bundle via webpack IgnorePlugin
Pt 7/7 - Unwrap validateMnemonicWordList

Test Plan:
npm test
npm start
create a new wallet and send/receive XEC to it
import an existing wallet and send/receive XEC to it

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12482