Constrain verbs to GET in routes.

We shouldn't be responding to anything else.
Re-organise routes to group templates by type
Hook in Rack::Test in integration tests
Refactor routing to be simpler

It's no longer necessary to list each template explicitly in the routes
(or in the caches_page line)
Prevent direct access to partials.

And other param validation.
Add comment explaining regex intent
Merge pull request #229 from alphagov/routing_refactor

Refactor routing so it's not necessary to list all templates