Attachment file URLs should always point to the central media site

Inserting an image into a post from the Add Media button results in a URL that points to the current site (at least when subdirectory sites are used).

The image URL ideally should always point to the central media site.

Stretch goal: The URL behaviour could be configurable in case this behaviour needs to controlled by the network. Need to look into the SEO benefits of each option. Need to test with:

* [ ] Subdirectories
* [ ] Subdomains
* [ ] Domain mapping

Start by reproducing image insertion from the Add Media button in the listed subdirectory, subdomain, and domain-mapping configurations. Trace where the attachment URL is generated and verify that it points to the central media site in each case; any configurable behavior and SEO trade-offs are identified as a stretch goal.

Written by the indexing model from the issue text.