Validate safeTxHash against tx data

[katspaugh]

@Uxio0 suggested a security enhancement:

• When displaying a transaction's details from the backend, compare a generated safe tx hash from the transaction data with the safeTxHash returned from the backend.
• If the hashes don't match, show a warning to the user and don't allow signing this transaction.
• Track these cases to Sentry as critical (if it ever happens)

This would detect hacking attempts on the backend.

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N.B. we currently never sign a raw safe tx hash from the backend and instead always generate a new hash from transaction data, so the user always signs what they actually see. However, this enhancement would be still useful to detect txs that could have been messed with.