Fix taxonomy not loading when using site subdirectory

[jasonvarga]

If you have a site that uses a subdirectory (like /subdirectory/ or /blog/ or /statamic/) then it would incorrectly prefix that to the taxonomy's uri, which should be relative to the site root.

When you visit /subdirectory/tags, it would look for a taxonomy with a uri of /tags, which doesn't match what Statamic thinks the taxonomy's uri is. This results in a 404. A similar thing happens for the term url.

The taxonomy url would also have the double prefix. e.g. /subdirectory/subdirectory/tags.