T2730
Stacked diff to localize existing uses of BCH.Mnemonic throughout the app and deprecate the BCH.Mnemonic object.
Relies on the BIP39 library for mnemonic operation.
**Pt 1/5 - Implement BCH.Mnemonic.toSeed locally**
Pt 2/5 - Implement BCH.Mnemonic.generate locally
Pt 3/5 - Implement BCH.Mnemonic.wordLists locally
Pt 4/5 - Implement BCH.Mnemonic.validate locally
Pt 5/5 - Deprecate BCH.Mnemonic throughout the app