Check list step to recreate keystore from mnemonic

It would be nice to add a check list step to recreate the keystore and deposit key using the same mnemonic.

## On the generate keys page

On https://launchpad.ethereum.org/generate-keys, I would add the following after
> ⬜ I am keeping my key(s) safe and have written down my mnemonic phrase.

⬜ I have recreated my keystore and deposit keys using my mnemonic and I have confirmed that they all share the same public key(s).

## On the checklist page

On https://launchpad.ethereum.org/checklist, I would add the following somewhere:

⬜ I have recreated my keystore from my mnemonic and I have confirmed that all the keystores I created share the same public key(s).

Those added check list step could have save someone's deposit already and hopefully they can help save someone else's deposit in the future.