Proxy captions via new /proxy route

We can add more authorized paths to authorizedPaths if we need
more resources to be pulled from the NewLeaf/Invidious backend
on the same domain.

This route forwards to the client a few headers like Bibliogram
ea7cd5d5/item/src/site/api/proxy.js (L28-29)
so we can also use this route to possibly proxy videos in the
future.

We are strict about the url parameter not overriding the
NewLeaf/Invidious backend (instanceOrigin) by ensuring
fetchURL has instanceOrigin as prefix.

api/video.js

@@ -174,6 +174,11 @@ module.exports = [
 				// rewrite description
 				video.descriptionHtml = converters.rewriteVideoDescription(video.descriptionHtml, id)
 
+				// rewrite captions urls so they are served on the same domain via the /proxy route
+				for (const caption of video.captions) {
+					caption.url = `/proxy?${new URLSearchParams({"url": caption.url})}`
+				}
+
 				return render(200, "pug/video.pug", {
 					url, video, formats, subscribed, instanceOrigin, mediaFragment, autoplay, continuous,
 					sessionWatched, sessionWatchedNext