Use Rumoji

Instead of using our own conversion table, let’s use Rumoji to convert emoji into text codes.

What is the purpose of Emojimmy now that Rumoji handles the conversions?

1. We still have the mixin that adds the `stores_emoji_characters` to automatically convert attribute values
2. After converting to text codes with Rumoji, we still trim characters that cannot be inserted in `utf8` MySQL fields. For instance, iOS sometimes insert `"\xEF\xB8\x8F"` characters between emoji. We remove those.