Add method to import a seed phrase from an external source

[westonb7]

Is your feature request related to a problem? Please describe.
If we want to integrate with MetaMask, we will need a way to accept seed phrases from external sources, at least in the WebGL version.

Describe the solution you'd like
Importing a seed phrase from an external source should allow someone to begin a learn mode run with the seed phrase being imported. This should bring up the 'encode seed' canvas of the start menu, and allow the user to proceed as normal from there.

Additional context
Related: MetaMask/metamask-extension#7364