LoginViewController uses dismissBlock after successful login.

`cancelButtonDisabled` flag added to LoginViewController so it doesn't depend on the existence of dismissBlock anymore to decide whether to show the cancel button or not.
cancelButtonDisabled flag changed to cancellable in LoginViewController
Merge pull request #2883 from wordpress-mobile/issues/2882-use-dismissBlock-in-login-page

LoginViewController uses dismissBlock after successful login.
Merge remote-tracking branch 'origin/develop' into issues/2755-tabbar-enhancement

Conflicts:
	WordPress/Classes/ViewRelated/Settings/SettingsViewController.m