The way our Local Site storage has worked in the past is that the URL
patterns would capture the Local Site's name, and the decorators for
views would turn that into a LocalSite object, passing it to the view.
This worked pretty well, but if other code needed access to the
LocalSite and only had the request, they'd have to refetch it based on a
private attribute on the request.

This change adds a public attribute on the request containing the
LocalSite itself. This attribute is only loaded on demand, the first
time it's accessed, and is None if not working with a LocalSite. The
decorators and other code can access this without worrying about
redundant fetches, and without accessing private attributes.

There's minimal impact to the Local Site code here. It's really just a
new convenience for code that needs a Local Site from a request (which
will be used in the upcoming integrations changes).

Tested various pages with and without a Local Site.
Tested this new attribute through the decorator and also in some new code

that only has access to the request.
Unit tests pass.