Update Jetpack_SSO to be a proper singleton

Remove usage of ‘new Jetpack_SSO’ in favor of Jetpack_SSO::get_instance
to emphasize that Jetpack_SSO is a singleton. Made the constructor
private.
Multisite: Partially revert 5ee811895d1138aebc046e8001b5b296d235807e

See #382: that class is meant to check non network active plugins.
Merge pull request #134 from bbrooks/sso-singleton

Update Jetpack_SSO to be a proper singleton
Push Notifications: Remove module extra info.

Resolves #175
Merge remote-tracking branch 'upstream/master'
Add currently connected wpcom user to debugger fixes #220