External links internal host needs to allow dynamic host

Currently, the default value is set to whatever the current host is. When developing locally, this could be something like markdown.ddev.site, which isn't the actual host for the production site. However, this value does get saved to config.

Instead, we need a way to dynamically replace a token with the current request's host.

Because this token doesn't exist in core, yet, we'll need to supplement this temporarily.