Add support for "is row checkable" in grids

i.e. when grid has checkboxes, some rows maybe shouldn't get one

tailbone/views/master.py

@@ -451,6 +451,7 @@ class MasterView(View):
             'url': lambda obj: self.get_action_url('view', obj),
             'checkboxes': checkboxes,
             'checked': self.checked,
+            'checkable': self.checkbox,
             'clicking_row_checks_box': self.clicking_row_checks_box,
             'assume_local_times': self.has_local_times,
         }
@@ -2765,6 +2766,7 @@ class MasterView(View):
     def get_effective_query(self, session=None, **kwargs):
         return self.get_effective_data(session=session, **kwargs)
 
+    # TODO: should rename to checkable?
     def checkbox(self, instance):
         """
         Returns a boolean indicating whether ot not a checkbox should be