Set default blog when syncing blogs for Today Widget

Closes #2546 

Sets the default blog for all accounts when syncing sites.  Configures the Today Widget if a default account for .com is available.

This PR will cause the Core Data store to be erased and login window presented.  Instead of creating adding a new model version, I chose to update v21 to be consistent with develop currently.

Do we want to throw in a `WIDGETS_EXIST` check here?