Bring credentials into the Riff-Raff app / DB

[sihil]

Credentials are currently split between an unencrypted ssh key on disk, secret keys held in the configuration file and account details looked up in Prism or DeployInfo.

This should:

• ideally be integrated into Riff-Raff itself and added to the configuration menu.
• consider challenges for the CLI (although we currently don't use that with any credentials)
• ensure that secrets are encrypted at rest in the mongodb (with the application secret? although that's stored in GitHub at present)