Refactor wiring

Some progress towards cleaning up the wiring file:

- Removing wiring instances that weren't used or were very simple indirections
- Not injecting some of the more stable dependencies (Panopticon API adapter, `Plek`, `Govspeak::Document`)
- Moving manual services into a `ManualServiceRegistry` which is constructed in the `ManualsController` to make it easier for newcomers to orient themselves (anecdotally, most people seem to start exploring a Rails app from the controller relevant to their current task)

I'd like to continue to separate services into one registry per controller and add a `RepositoryRegistry` to simplify using repositories without having to know all of their dependencies.