BUG: TACHYON_URL broken image paths for certain subdirectory networks. #761
Description
Depending on the configuration of a network site, the TACHYON_URL constant can be incorrectly set.
If the main site (site id 1) URL is a subdirectory, e.g https://example.com/subsite1/ the TACHYON_URL image URLs will break, as the URL will include subsite1, but the image location would be stored in root, or possibly using the site ID.