Incorrect filter handler for "Signup: Reminder: Enabled/Disabled"

The "Signup: Reminder: Enabled/Disabled" filter is broken, since it's trying to use "views_handler_field_boolean_operator" which doesn't exist. At least we need views_handler_filter_boolean_operator there, but in light of #336450: Signup's views2 boolean filters have terrible UI we should probably write our own handler class for this to extend views_handler_filter_boolean_operator and make the UI reasonable for this use.