Changed tests to reflect new desired behavior for jars_in_stock.

I am changing the jars_in_stock method to return a number of jars, not
a list of jars.  First stop, changing the test.
Changed jars_in_stock() to match tests.  Ref #2.

Both jars_in_stock() and first_available() now use Jar.instock.  There
is probably an elegant way to reference
Jar.instock.filter(product_id=self.pk) but I don't know how yet.  I've
left a comment to remind myself to fix it.
Changed catalog admin page.  Refs #2.

Changed product admin page to sort by create date and activity, also
enabled filtering by category and activity.  Changed category page to
show count of products instead of list of products.  Could not do the
same change as was done for products due to the product value being
used elsewhere.
Added link to products from category page.

The admin site for categories now has a link to the list of products
in that category.
Orders and pick lists now share links.  Ref #2.

Each order with an uncancelled pick list has a link to that pick list.
Each pick list has a link to its parent order.
Fixed product ordering and added inventory drilldown.  Fixes #2.

The product ordering has been changed to push inactive products to the
bottom and sort from newest to oldest.  The link approach used for
categories has been applied to inventory and provides a drill-down
from the warehouse level all the way to the crate.

This completes all of #2 that can be fixed without the ability to
properly set a default filter.
Merge branch 'feature/issue-2' into develop