Support IP address detection from X-Forwarded-For

[brianhelba]

This header is set by Heroku and other reverse proxies.

Respecting its value is critical to ensuring that HttpRequest.META.REMOTE_ADDR is set properly (which in turn, impacts things like Sentry error reporting).

When the header contains a list of addresses, it's important that the last element be used; see this and this.

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Several third-party options for a fix:

• https://github.com/allo-/django-xforwardedfor-middleware ; uses the first element in a list, rather than the last
• https://github.com/ferrix/xff/ ; perhaps too feature-bloated