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

pagefind/integration_tests/web_components/relative-image-urls-base-url.toolproof.yml

@@ -0,0 +1,49 @@
+name: Web Components Tests > Image URLs are resolved against an absolute base URL
+steps:
+  - step: I have the environment variable "PAGEFIND_SITE" set to "public"
+  - step: I have a "public/index.html" file with the content {html}
+    html: |-
+      <!DOCTYPE html><html lang="en"><head>
+      <link href="/pagefind/pagefind-component-ui.css" rel="stylesheet">
+      </head><body>
+      <pagefind-config preload base-url="https://example.com/"></pagefind-config>
+      <pagefind-input></pagefind-input>
+      <pagefind-results show-images></pagefind-results>
+      <script src="/pagefind/pagefind-component-ui.js" type="module"></script>
+      </body></html>
+  - step: I have a "public/blog/post/index.html" file with the content {html}
+    html: >-
+      <!DOCTYPE html><html
+      lang="en"><head></head><body><h1>Relative Image</h1><img
+      src="images/header.png"><p>A page with a relative image
+      URL.</p></body></html>
+  - step: I have a "public/about/index.html" file with the content {html}
+    html: >-
+      <!DOCTYPE html><html
+      lang="en"><head></head><body><h1>Absolute Image</h1><img
+      src="/images/hero.png"><p>A page with an absolute image
+      URL.</p></body></html>
+  - macro: I run Pagefind
+  - step: stdout should contain "Running Pagefind"
+  - step: I serve the directory "public"
+  - step: In my browser, I load "/"
+  - step: In my browser, I click the selector "pagefind-input input"
+  - step: In my browser, I type "image"
+  - step: In my browser, the console should be empty
+  - step: In my browser, I evaluate {js}
+    js: |-
+      let results = await toolproof.querySelector("pagefind-results ul");
+      await toolproof.waitFor(() => results.querySelectorAll("li a").length === 2);
+  - snapshot: In my browser, the result of {js}
+    js: |-
+      let results = await toolproof.querySelector("pagefind-results ul");
+      let items = Array.from(results.querySelectorAll("li"));
+      items.sort((a, b) => a.textContent.localeCompare(b.textContent));
+      return items.map(li => {
+        let title = li.querySelector("a").textContent;
+        let img = li.querySelector("img");
+        return title + ": " + (img ? img.getAttribute("src") : "no image");
+      }).join("\n");
+    snapshot_content: |-
+      ╎Absolute Image: https://example.com/images/hero.png
+      ╎Relative Image: https://example.com/blog/post/images/header.png