local_context_processors: Add host_url.

This makes it easier for templates to generate a "single canonical URL" for
a resource, as in Open Graph metadata.

Add sitefundgoal to top level view context.

The base template now has access to the fund-raising goal that will be
displayed site-wide. The view here must provide it in the context for
access everywhere.

Furthermore, we need a local context processor to make sure it's in
context everywhere, and we have to make sure that context is properly
passed along.

This code is a bit cargo-culted in, based on what I read in various
places about adding the Request context.