Extract id_generator wiring into a class
Add ManualServiceRegistry

A registry containing just the services for manuals is simpler to reason
about and has fewer dependencies.

By constructing it in the controller we make the codebase easier to work
with for newcomers whilst keeping the benefits of injecting
dependencies.
Remove manual services from ServiceRegistry

Now that we have a ManualServiceRegistry we don't need them.
Get rid of slug_generator wiring

Instantiating a new SlugGenerator is simple enough to do inline in the
three places that care about it.