Visual editor migration

[maxme]

Note: there is no changes in the DB, but since we have all other migrations done there I added the visual editor migration there.

1. old users upgrading from any versions to this version should have the new editor available and enabled
2. new users should have the new editor available and enabled
3. users who previously disabled the new editor should have it enabled
   WordPress.com Notifications in Android #3 is about our future announcement "hey check out the new editor" and I guess these users are rare.

[aforcier]

Confirmed that upgrading from vanilla and clean installing this branch enables the editor properly.

As we discussed, #3 should affect few enough users that it's not worth adding a special rule for.

🎉