Prompt for password change when using 'jrnl --encrypt' on already encrypted journal

[jonakeys]

This PR addresses issue #1358 Using --encrypt on an already encrypted journal now warns the user the journal is already encrypted and prompts for creating a new password.
I also added a BDD test.

No config file changes were needed as far I can see. But I'm not sure if the BDD test is complete this way or if it should need to enter a new password and check if the password indeed changed on the re-encrypted journal.

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed.

[wren]

Nice! Thank you for implementing the @todo test!