Bug 735311 - Adapt to new Google HTTP restriction

Google has recently imposed a limit on the number of unauthorized HTTP
requests to its OAuth2-based interfaces.

The normal operation of SoupSession is to issue the first HTTP request
unauthorized and see if the server issues a "401 Unauthorized" response
(since not all HTTP services require authorization).

On a "401 Unauthorized" response, SoupSession emits an "authenticate"
signal, which the application is supposed to handle.  Once credentials
are in hand, SoupSession resends the same HTTP request and subsequent
requests with an Authenticate header while the socket connection holds.

Google's unauthorized request limit breaks this logic flow.  Once the
limit is exceeded, the Google server issues a "403 Forbidden" response
instead of "401 Unauthorized" to an unauthorized HTTP request.  This
breaks error handling in the E-D-S CalDAV backend.

The workaround is to preload the SoupAuthManager with a pre-configured
SoupAuth when using OAuth 2.0 authentication.  This avoids the initial
unauthorized HTTP round-trip.

The backend implements the GInitable interface for this since obtaining
a valid access token is a failable step.