Eliminate placements show

This ended up being a beast of a branch because it has now accumulated the work of three features: remove takeaway from a rack, retire a takeaway, and get rid of the placement show page.

We talked about getting rid of tables when we moved the stocking list from the placement show page to the takeaway & rack show pages.  However, I think if we want to do that, it should be a separate task.  I've left the tables in place for now & just re-styled slightly.
Not super happy with this, but I tried for a long time to get this result without resorting to raw SQL (without success obviously).