zulip-icons: Disable ligature generation.

This makes our icon font smaller.  We only reference its glyphs by
Unicode private-use codepoints and not by ligatures.

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

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

@@ -6,4 +6,5 @@ module.exports = {
     classPrefix: "",
     baseSelector: ".zulip-icon",
     cssTemplate: "./template.hbs",
+    ligature: false,
 };