emoji_codes: Replace JS module with JSON module.

webpack optimizes JSON modules using JSON.parse("{…}"), which is
faster than the normal JavaScript parser.

Update the backend to use emoji_codes.json too instead of the three
separate JSON files.

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

static/js/reactions.js

@@ -1,3 +1,4 @@
+const emoji_codes = require("../generated/emoji/emoji_codes.json");
 const render_message_reaction = require('../templates/message_reaction.hbs');
 const Dict = require('./dict').Dict;