Gate SIGHASH flags under an app setting

Currently, we simply give a warning when a transaction contains non-standard SIGHASH flags.

Since it's a niche feature not used by many users, and it's generally not safe to ignore the warning, we could consider rejecting them by default, and adding a setting (persisted in NVRAM) to enable them, so that only users that enable the setting can sign such transactions. (The warning will remain).