Guard against invalid SSL certificates

None of the branded providers (eg., Google, Facebook and Windows Live)
should ever have an invalid certificate. So set "ssl-strict" on the
SoupSession object being used by GoaWebView.

Providers like ownCloud and Exchange might have to deal with
certificates that are not up to the mark. eg., self-signed
certificates. For those, show a warning when the account is being
created, and only proceed if the user decides to ignore it. In any
case, save the status of the certificate that was used to create the
account. So an account created with a valid certificate will never
work with an invalid one, and one created with an invalid certificate
will not throw any further warnings.

Fixes: CVE-2013-0240