Improvements to slug redirection

This is required for fixing malformed slugs for #1449 so that we
don't break old links. Previously SlugRedirect was only used for
redirection in PersonDetailView, but this adds it for organisation
and place views, including the sub-pages.

It also adds stricter validation of slugs in the admin interface so
that people can't accidentally create slugs with capital letters in
them.

Finally, this moves all the code for dealing with slug redirections
into its own Django application.