We had a lot of code that attempted to build a server URL by fetching a

Site, SiteConfiguration, doing a URL lookup, and building a string. This

was getting silly, so I've added two central functions for this purpose:

get_server_url() and build_server_url().
get_server_url() returns the URL to the root of the server (or a Local

Site, if one of the identifying arguments are provided).
build_server_url() builds a URL off of the server URL. It's a

convenience around calling urljoin(get_server_url(), ...).
Many call sites have been updated to use these new functions. Many still

remain, as we have templates that are building their own URLs, and some

code that's doing fancier URL building, and I didn't want to risk

breaking them right now.
This also fixes a bug with the e-mail headers. We were always referring

to the root of the site, rather than the Local Site, which made it harder

to filter when working on multiple Local Sites.

Unit tests pass.
Tested each affected call site manually, with and without Local Sites.