Export identity keys with passphrase

[erangell]

In order to backup identities or transfer them to another machine there needs to be a mechanism for the user to export their private keys and import them to another instance of Beaker. Maybe a UI allowing the user to review all their identities and Export/Import one or more of them to an ascii armor file would work. Prompt the user to create a passphrase and explain that the file contains their private keys so they need be careful where they store it and use it.

[pfrazee]

This is also difficult because there's currently a "corruption" risk when moving dats between devices. See dat-ecosystem-archive/DEPs#43 for discussion.