Local: ensure template assets use the correct port locally

[timmywil]

While debugging local setups with Andre, I noticed that when I changed port, the template assets continued to be served from port 80. This filter ensures that theme_root_uri includes the local port number, even after a database is populated with a portless siteurl, without the need to alter the database.

For whatever reason, Andre didn't have this issue when switching ports, but I did, and I couldn't tell you why this isn't always necessary.