[PD-841] User-proofed expiration date

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
* added a small helper to universal helpers to make digging inside of nested structures for a value easier. Something tells me this is already available somewhere (`[foo[0] for foo in bar].index('baz')`) isn't quite as flexible or efficient as `find('baz', foo, 0)`).