zulip-icons: Set writeFiles to false.

webfonts-loader now defaults writeFiles to true, which makes spurious
copies of zulip-icons.{css,eot,svg,ttf,woff,woff2} for no reason.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

static/assets/icons/zulip-icons.font.js

@@ -7,4 +7,5 @@ module.exports = {
     baseSelector: ".zulip-icon",
     cssTemplate: "./template.hbs",
     ligature: false,
+    writeFiles: false,
 };