Make Jitsi download script a JS script

It's awful but means that we create the directory on all platforms in-process.

Fixes https://github.com/vector-im/riot-web/issues/13215

scripts/build-jitsi.js

@@ -0,0 +1,22 @@
+// This is a JS script so that the directory is created in-process on Windows.
+// Sorry.
+
+const fs = require("fs");
+const path = require("path");
+const mkdirp = require("mkdirp");
+const fetch = require("node-fetch");
+
+console.log("Making webapp directory");
+mkdirp.sync("webapp");
+
+// curl -s https://jitsi.riot.im/libs/external_api.min.js > ./webapp/jitsi_external_api.min.js
+console.log("Downloading Jitsi script");
+const fname = path.join("webapp", "jitsi_external_api.min.js");
+fetch("https://jitsi.riot.im/libs/external_api.min.js").then(res => {
+   const stream = fs.createWriteStream(fname);
+   return new Promise((resolve, reject) => {
+       res.body.pipe(stream);
+       res.body.on('error', err => reject(err));
+       res.body.on('finish', () => resolve());
+   });
+}).then(() => console.log('Done with Jitsi download'));