PSM: Reset 'Published Date'

Closes #4557
- The underlying issue is that the PSM is already loaded when Ember
routes from an existing post to a new post. Instead of resetting the
‘Published Date’ value manually (with jQuery), I’m using Ember’s
computed property setter to ‘refresh’ the cache for that property. I
believe that this Ember solution is better than manually going in and
resetting it with jQuery.
I mean, I don't really want to use the input (and this property shouldn't *really* have a setter), but a setter is a really clean way of poking the cash for that property.