mobile sharing: Make emoji.js a shared ES6 module.

This is a pretty straightforward conversion.

The bulk of the diff is just changing emoji.js
to ES6 syntax.

There is one little todo that can be deferred
to the next commit--we are now set up to have
markdown.js require emoji.js directly, since
it is no longer on `window`.

static/js/ui_init.js

@@ -1,4 +1,5 @@
 const generated_emoji_codes = require("../generated/emoji/emoji_codes.json");
+const emoji = require("../shared/js/emoji");
 
 const emojisets = require("./emojisets");
 const markdown_config = require("./markdown_config");