points_valid flag for events

[JustinSGray]

Events needs to have a points_valid flag.

-If false (default), even if results are uploaded to the site, they wont display on views.
-if true, results are shown
-any time a run, result, or registration is changed, the the flag needs to be invalidated. This should be done with django signals

signal should have one method to invalidate the flag, and (maybe) recalculate the points, if it was true when the method is called. All other things should just call that signal method. This makes things very flexible, and gives me some experience with signals.