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.

These are probably better off as `NSParameterAssert`. See http://nshipster.com/nsassertionhandler/ - particularly this blurb:

>The first distinction is between a general assertion (NSAssert) and a parameter assertion (NSParameterAssert). As a rule of thumb, methods / functions should use NSParameterAssert / NSCParameterAssert statements at the top of methods to enforce any preconditions about the input values; in all other cases, use NSAssert / NSCAssert.