Exclude some paths from being redirected

[chillpop]

Allow setting excluded paths that will never be redirected.

• Adds a setting to set excluded paths
• Warns when creating/editing a redirect about matching excluded paths
• Updates for more modern Django versions

[emperorcezar]

I think you'll need to clear the url caches in the Redirect save to make it fully compatible with latest Django.

Also some minor comments.

[emperorcezar]

Gonna go ahead and approve, but why not put it in https://github.com/spothero/django-robust-redirects/blob/master/robustredirects/utils.py ?