Allow creating a wallet with 24-word recovery phrase

[gabaldon]

24-word recovery phrase is already supported, the UI need to allow selecting the preferred recovery phrase length.

[aesedepece]

As this isn't the most popular feature in the world, I'd be OK with partial support, i.e. allow importing 24 words, but not creating them (the user will need to create it with any other software). In that way we don't clutter the standard wallet creation flow for the 99% of the users with a feature that will only be appreciated by that other 1%.

[parodyBit]

the user can already import mnemonics of length 12, 15, 18, 21, and 24.