Fixes for Ticket 4120 - JS cleanup for admin

This was quite an old ticket, but most of the original patch still applies. Consider it spring cleaning?! 

It is just js cleanup. No admin functionality. The most import change is that it removes a duplicative function from calendar.js, which is also found in core.js. 

The other two commits shuffle a couple functions into core.js. This ticket seemed to be a prerequisite to addressing [#4045](https://code.djangoproject.com/ticket/4045) which I will pick up once a call is made on this ticket.

The only part of the original patch that didn't apply were the changes to js/admin/ordering.js, which appear to have been removed. This change looked to be removing a duplicative `addEvent` function.

[Associated TIcket](https://code.djangoproject.com/ticket/4120)