Move `SITE_FUNDGOAL` configuration to settings/base.py

This changes more often than the surrounding code, so should live with the other
configuration.

contacts: Remove ContactEntry and add Unsubscription

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.

Remove summit_registration Django app

Has some typos suggesting that the code was never completed or used.

Move the content templates/files into `conservancy/content`

These were previously intermingled with the static content in `conservancy/static`.

Delegate management of canonical URLs to Apache

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".

Switch settings to use "the one true way" approach

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.