Generate key pair via p2panda-js and store it locally

[cafca]

Todo:

• Import p2panda-js package
• Generate new key pair when it does not exist yet
• Restore key pair when it already exists
• Optional: Handle key pair in separate file which can be accessed globally (can be later improved with Higher-order component for p2panda state #14)

Steps to handle key pair:

• Check if private key exists in localStorage
• If it does not exist, generate new key pair via const keyPair = new KeyPair(), store it in localStorage
• If it exists, restore key pair via const keyPair = KeyPair.fromPrivateKey(privateKey)

[cafca]

We already implemented everything asked for here