Makes Deployment and DeploymentState many to many.

Closes #7

Especially adds pending credential states

The user of a NewDeployment can be None

Previously if a service had no sites and no site was manually given
create_state_items failed. We now ignore such services.

A large step towards solving #7

Alongside other linting / minor changes.

Related: #7

This to enable the individual key handling needed for #7.