Allow local_site_reverse to take an actual LocalSite.

local_site_reverse was able to take a LocalSite's name, or a request
object, but if you actually had a LocalSite (or None), you'd have to
write your own conditional to extract the name and pass it.

Now, local_site_reverse can take a LocalSite. This saves a database
query, if one is already available, and simplifies calling code.

Reviewed at https://reviews.reviewboard.org/r/6302/