Simplify persistence and registraion of routes.

Given what's being persisted on the model is in the same format as the
data passed in, we can just store that like any other field, and then
have the route registration act on that field.
Refactor tests for ContentItem registering routes

Removes some duplication
Clarify route registration assertions.

This was previously asserting that 2 requests were made because one was
triggered when the factory was created.  This wasn't immediately
obvious.

This changes the test to clear out WebMock's registered requests after
the factory creation to make the assertion clearer.
Refactor assert_routes_registered test helper.

This removes some of the knowledge of router-api paths etc from this
app, and relies on gds-api-adapters for this.

Note: this relies on an unreleased version of gds-api-adapters to be
added in a future commit
Register a backend when registering routes.

The router-api will reject routes pointing at a non-existent backend.
This therefore creates a backend before creating routes.  This is
consistent with other places in our stack where we're registering
routes.
Bump gds-api-adapters to 11.0.0

To pull in needed test helpers.  Also remove the skip_commit option on
the register route call as this is now the default behaviour.
Move ENV setup into spec_helper.rb

Previously, the tests were dependent on the spec/support files being
loaded in a order (specifically, the env setup had to be loaded before
the router helpers).  This order is not guaranteed, so moving the env
setup into the spec helper will ensure it's done before the router
helpers are loaded etc.