fix: use createFromUri instead of addFromUri for keys

[JoeCap08055]

This PR changes our usage of keyring.addFromUri() to keyring.createFromUri() so that keypairs will not be saved in a long-lived variable.

Note, this is only a minor improvement to security for the following reasons:

1. We do not have control over JS garbage collection, so not way to guarantee that the generated keypair is eliminated from memory at any given point in time
2. The seed phrase used to generate the keypair is currently injected to the application as an environment variable, so anyone with enough access to view the process could also see the environment

Closes #187

[wilwade]

One suggestion to move new Keyring({ type: 'sr25519' }) to a const at the top level

[wilwade]

👍