Support 24-word (256 bit) BIP39 mnemonic strings

[lemois-1337]

What it is?
Similar to the open issue karlsen-network/karlsend#27 which will add 12-word support into the karlsend node daemon, we must support 24-word mnemonic strings in the web wallet as well.

Why it is needed?
It would be very beneficial for the user to specify the entropy (bit size) by selecting 12/24-word seed phrase during wallet creation and/or import to make it portable and allow using it in paper wallet, mobile wallet, karlsend node wallet and Node.js CLI wallet. At the end we will support 12- and 24-word mnemonics in all wallet implementations to get full interoperability.

Additional Information
https://github.com/bitpay/bitcore/tree/master/packages/bitcore-mnemonic

[mahalamobile]

yeah need web wallet to support 24 seed phrase please;)