Remove govspeak lambdas from wiring.

The "SpecialistDocumentGovspeakToHTMLRenderer" and "HeaderExtractor"
classes both already have (in essence) a hard dependency on Govspeak.
They're also not used to render anything other than govspeak.

By pushing the dependency on govspeak down to these classes from the
wiring, and using simple dependency injection for the tests, we can
further simplify the wiring.
Push SecureRandom dependency down to the builder

SecureRandom is part of ruby corelibs -- I don't think we need to
explicitly test it, just use simple DI to make it testable.

Allows us to further simplify the wiring.
Prefer double-quoted strings
Depend on an abstraction of SecureRandom.

Per a PR comment by @heathd let's depend on an abstraction rather than
secureRandom itself.