Resolve result images against an absolute baseUrl

Relative images were resolved against the page URL, but only the
pathname was kept, so with a baseUrl on another domain the image
still loaded from the search site. Keep the origin when the page URL
has one, in the Component UI's resolveUrl filter and in both Default
UI result templates. result_with_subs.svelte didn't resolve images at
all before.

Fixes #573

docs/content/docs/components/results.md

@@ -88,10 +88,11 @@ The template engine provides [built-in filters](https://github.com/bglw/adequate
 
 #### `resolveUrl(pageUrl)`
 
-Resolves a relative URL against a page URL. Absolute URLs are returned unchanged.
+Resolves a relative URL against a page URL. Absolute URLs are returned unchanged. If the page URL includes a domain, for example from a `baseUrl` on a merged index, the resolved URL keeps that domain.
 
 ```
 "images/hero.png" | resolveUrl("/blog/post/")  →  /blog/post/images/hero.png
+"images/hero.png" | resolveUrl("https://example.com/blog/post/")  →  https://example.com/blog/post/images/hero.png
 ```
 
 ### Placeholder Template