Configure link fetch to use prefetchTimeout

This change modifies the `fetch` function in `link.js` to use the new `prefetchTimeout` config setting introduced in the previous commit. This allows configuring the length of the timeout.

I've added a comment here to indicate milliseconds are the unit in use, since otherwise that would no longer be obvious from the code without looking at the default value (which could change).

src/plugins/irc-events/link.js

@@ -390,7 +390,7 @@ function fetch(uri, headers) {
 		try {
 			const gotStream = got.stream(uri, {
 				retry: 0,
-				timeout: 5000,
+				timeout: Helper.config.prefetchTimeout, // milliseconds
 				headers: getRequestHeaders(headers),
 				https: {
 					rejectUnauthorized: false,