Better console UX for seed phrase generation

[plaidfinch]

This is in response to a long-outstanding comment in the pcli code that we should make it harder to accidentally spill your seed phrase when copying console output. This fixes that, as well as generally improving seed phrase ergonomics in other ways:

• Seed phrases are now displayed in a way that makes them easier to read when writing them down.
• Users are prompted to explicitly confirm that they have written down their seed phrase.
• Seed phrases are cleared from the screen after being written down, so they cannot accidentally show up in copied logs after the fact.

The interaction looks like the below...

1. Initially, you see this:

        Welcome to Penumbra. 🌘 

  Below is your private seed phrase.
    No other person should ever read it.
  It cannot be recovered if lost.
    It will never be displayed again.

Write it down right now and keep it safe!

 1. execute   7. carry    13. shield   19. loan    
 2. confirm   8. mad      14. youth    20. combine 
 3. maximum   9. floor    15. husband  21. opinion 
 4. choose   10. frozen   16. vessel   22. solution
 5. now      11. amount   17. fly      23. vibrant 
 6. attract  12. sport    18. shallow  24. oval    

To continue, type:

  I have written down my private seed phrase.
> █

2. Then, as you're typing the confirmation prompt, it looks like this:

        Welcome to Penumbra. 🌘 

  Below is your private seed phrase.
    No other person should ever read it.
  It cannot be recovered if lost.
    It will never be displayed again.

Write it down right now and keep it safe!

 1. execute   7. carry    13. shield   19. loan    
 2. confirm   8. mad      14. youth    20. combine 
 3. maximum   9. floor    15. husband  21. opinion 
 4. choose   10. frozen   16. vessel   22. solution
 5. now      11. amount   17. fly      23. vibrant 
 6. attract  12. sport    18. shallow  24. oval    

To continue, type:

                             ate seed phrase.
> I have written down my priv█

3. Finally, when you're done, it censors the seed phrase in the console:

        Welcome to Penumbra. 🌘 

  Below is your private seed phrase.
    No other person should ever read it.
  It cannot be recovered if lost.
    It will never be displayed again.

Write it down right now and keep it safe!

 1. ████████  7. ████████ 13. ████████ 19. ████████
 2. ████████  8. ████████ 14. ████████ 20. ████████
 3. ████████  9. ████████ 15. ████████ 21. ████████
 4. ████████ 10. ████████ 16. ████████ 22. ████████
 5. ████████ 11. ████████ 17. ████████ 23. ████████
 6. ████████ 12. ████████ 18. ████████ 24. ████████

Saving backup wallet to /home/$USER/.local/share/penumbra-testnet-archive/f1b1dd6d7b830d0e/custody.json