[PD-841] Change expiration date representation

I tried to comment what I changed. This replaces the custom date widget with a duration drop down. I need to look at my old PR to see what the label needs to say. 


##From #1066:
For purchased jobs, the expiration date shouldn't exceed the posting length dictated by the product.
* changed to an integer drop down with 15, 30, 60, and 1 year posting time frames
* made sure only options valid for the particular product show up
* made the longest posting duration the default on initial form load
* edit remembers last date picked
* should honor original post date of job to prevent perpetual editing of an almost expired job