This reverts commit ff0ea068de.
The next commit (Create a slug for old orders in the migration) will
change our strategy of writing code to handle the legacy slugless orders:
we will generate a slug for old orders.
Orders created before we introduced slugs don't have a slug. This commit
introduces code to work with them. Without these changes, the legacy
orders are not reachable any more, and trying to create a link for them,
crashes the page.
I wrote this commit because in my test environment I had a long-lived
order for testing purposes, and the home page crashed because the order
would show up in the list of Open Orders.
