Get rid of the simplest wiring instances

These are providing some intermediate indirection without adding much
value.
Remove unused wiring
Don't pass api_client into PanopticonRegisterer

The API client and the way it's constructed is a very stable part of the
codebase. As it's likely to change significantly less frequently than
the PanopticonRegisterer itself needs to change, it's fine to rely on
the concrete details inside the class.

The advantage of doing this is that it reduces indirection (it's simpler
to follow what's happening without dependencies being passed in) and it
removes some noise from the wiring file.
Remove url_maker from wiring

`UrlMaker`s only dependency is Plek, which is both easy to construct and
stable, so make it build that for itself.

Since `UrlMaker` is only used in one place and now doesn't need any
arguments to construct it, just build it directly in the
application_controller.
Move inline simple single-use factories
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.
Merge pull request #162 from alphagov/refactor-wiring

Clean up wiring (a bit)