Encrypt accounts with backup password

[jlcvp]

Description

The app should be able to optionally encrypt/decrypt the secrets of the accounts with a user password without.
Users must be warned the accounts will be lost if they lose the password.

Subtasks

• Encrypt/decrypt logic
• Import/Export handling
• Ask for password on account loading
• Optionally save password on local storage
• Informational alerts when setting up the password
• Migration from password-less version
• Detect unencrypted base and ask the users if they want to encrypt the data