Run the account API without a Google client

With an empty Google client identifier or secret the API answered every
request with 500, including registration, password sign-in and the status
route. The Google scheme was always registered, and the authentication
middleware initializes every remote scheme for each request; with empty
credentials the validation of the Google options threw there, before any
endpoint ran. The API could not be started locally without credentials, and
the "Google sign-in is unavailable" answer of the start endpoint was never
reached.

The Google scheme is now registered only when both values are configured.
Without it the service runs normally, the start endpoint answers 503, the
completion endpoint redirects to the failed sign-in page without a session,
and the callback path is not a route. With both values nothing changes.

The test factory passes the two values as host settings. The program makes
the decision while it registers services, and configuration sources a test
factory adds are applied only when the host is built, after that point.

New tests run the service without a Google client: the ordinary routes,
a full register, sign-out and sign-in round trip, and the three Google
paths. They failed with 500 against the previous program and pass now, also
when the environment of the test run carries Google credentials.

The guides no longer list this as a limitation or ask for placeholder
credentials.

Deploying this change restarts the API, which deletes every account while
the store is in memory.

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