Fix #582 sync blog visibility

Fix #582 sync blog visibility for wpcom users - also split wporg and wpcom login / fetch blog list calls.
Bonus: for wpcom users, auto select primary blog on login.

This PR must be tested against different wpcom and self hosted configurations:

* "normal" self hosted
* self hosted + self signed SSL
* self hosted + HTTP Auth
* self hosted + Jetpack
* wpcom + 2FA
* wpcom account with no blog
* wpcom with exactly one blog
* wpcom with two or more blogs