Display a list of all conversations for an account

This PR adds:

- Log into a Helpful.io account
- Save the credentials securely on the device
- Fetch and list all accounts for the current user
- Fetch and list all conversations for a selected account

I haven't added tests for the models yet, since this requires a reliable way to get expected server responses. I'm still looking into this.

I also do not know what the `number` property on a `conversation` is supposed to mean. Can someone clarify?