Allow or restrict signing of unsupported transaction forms #20
Description
Right now the devices warns if an aspect of the transaction is "unsupported" (i.e. if there are unparseable parts of the Tx that can't be shown to the user for confirmation).
As a default condition, the device should refuse to even offer these transactions for confirmation, because the device cannot assure the user that tampering has not occurred. However, if the user really wants to sign these transactions, they should be able to, but it should require a specific enabling step.
The settings menu already contains a flag for "arbitrary data" that is for this purpose. However it is a holdover from the EOS wallet and is ignored in the current UX. This setting should be renamed to something more clear, and checked prior to entering the Confirmation Workflow for signing transaction. If "arbitrary data" is not allowed, then the UX should display a message and return to the main window without allowing for signing.
Related issue: #13 "Prominent warnings for unsupported transaction forms" — The conditions for warnings are similar to the conditions for disallowing signing, so this could be an alternate exit path following the warnings screen.
Changing the setting should require PIN code entry.
See also
Issue #14 "Refine 'Confirm Transaction' Workflow" — This issue may have more specifics on the UX handling of non-singable transactions.