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Closes #14

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.

Partly addresses #3

This resolves #2

The problem runs deeper than this

This prevents name clashes with old user instances. This was usually a
problem when the upstream IdP changed the subjects of our users.

The flow from here on is: When a user logs in with a changed subject
a new user is created. The old user remains (but is not accessable by the
user).