FileAttachment has a confusing relation to LocalSite. For historical

reasons, attachments which are related to a review request have a blank

local_site field, using the review request's local site instead. The

local_site field is therefore only "usable" for user file attachments

(such as files which are dropped into a comment box or review request

field).
This was causing a crash for review requests with attachments when

attempting to create the new download URL.
This change adds a fix for the URL crash, a big comment on the

local_site field explaining what's up, and some unit tests to ensure

that get_absolute_url() does the right thing.

Was able to view a review request with attachments on the new RBC

  server.
Ran unit tests.