Document that Submission Keys with Passphrases are not supported by SecureDrop Workstation

[nathandyer]

Although we recommend against setting a passphrase for the submission private key when setting up a SecureDrop Server, it is possible that one has been set by a user anyway. While this works for the classic SecureDrop Tails system on the Secure Viewing Station, trying to import a Submission Key with a passphrase set will result in an error when running sdw-admin --validate, and cannot be used by the Client.

We should add a note to the documentation explaining that Submission Keys with Passphrases are not supported.

Additionally, I recommend that we suggest that users reach out to support in the event that they have a submission key with a passphrase set so we can assist with exporting a version without a passphrase set (I feel these instructions are specific/obscure enough to not merit being added to the public documentation, but if others feel differently I'd be open to including them in the docs as well).

[zenmonkeykstop]

Is this a dupe of #95 ?

[nathandyer]

Ah yup - will close this one and add comments into the existing thread