SEC-977: Add support for CAS gateway feature

The opportunity and the implementation details of this new feature were discussed in Jira SEC-977.

The new filter TriggerCasGatewayAuthenticationFilter has been added to
call the CasAuthenticationEntryPoint when we want try a silent CAS
authentication (typically on a public page). The trigger criteria is
done with a requestMatcher instance.
The method unsuccessfulAuthentication has been overridden in
CasAuthenticationFilter in order to redirect to the saved url if there
was no SSO session (no service ticket sent from CAS).
To avoid infinite loop, we use the DefaultGatewayResolverImpl from Jasig
Cas Client.

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