Notifications: Changes to load the new notifications client.

Submitting mainly for discussion.

This sets a var `wpNotesIsJetpackV2 = true` to let the code loaded from wpcom in `admin-bar-v2.js` know to load the new notifications client. Also in the markup, the id of the panel needs to be `wpnt-notes-panel2` instead of `wpnt-notes-panel` to get the right css rules. Separate change to load the appropriate stylesheet if `is_rtl()`

I talked to @lezama and @rase- briefly about this. I'm not sure if you guys should ship to users in upcoming release, we have been testing the new client internally, but have not had a chance for larger scale testing yet - though because the code is loaded from wpcom it is possible for us to update later to fix any issues that come up.