Move HTTP basic auth into URLRequest
Move HTTP basic auth into URLRequest

The logic for adding HTTP basic auth headers to a request has been moved
into the `URLRequest` class since it is generally useful. In addition,
the `HostingServiceClient._build_request` method has been removed since
it was used in one place; its definition has been moved inline.

Testing Done:
Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/8911/

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