This change removes the unused `ContactEntry` model and the `subscribe` view and
replaces it with an `Unsubscription` model and an `unsubscribe` view. It works
similarly, but is intended to be used with the `list-unsubscribe` and
`list-unsubscribe-post` headers.
This middleware is mostly redundant:
* redirecting to canonical URLs can be done more simply in Apache
* appending a forward slash is a default in CommonMiddleware now
* we're no longer using Squid cache
May need to update Apache to strip/redirect trailing "index.html".
The advantage of this approach is that the production and dev configurations are
in version control, so there's less opportunity for surprises.
As advocated by Jacob Kaplan-Moss (OSCON 2011) and Two Scoops of Django book.
