[bug 1084387] Add on_picker field to Product

If a product should show up in the product picker, then on_picker=True.
If a product should not show up in the product picker, then
on_picker=False.

In this way, we don't accidentally reveal some products we need to
gather feedback for, but don't want to confuse users leaving feedback
for other products with their existence.
[bug 1084387] Add on_picker to management forms

* add on_picker to product management form
* add on_picker to product admin
* add enabled=True filter to Product.get_product_map() and also the list
  of products for the picker--we don't want disabled products showing up
  in either place
Merge pull request #369 from willkg/1084387-show-in-picker

[bug 1084387] Add on_picker field to Product