When we serialize the attributes of an awarded emoji, we ask the url of
the emoji. This will only be set for Custom Emoji, and to find it, we
need to look it up in the database.

Now for built-in emoji, we know all the names of those, and we know they
don't have an url set. So if the awarded emoji is one of the built-in,
early return without ever querying the database.

To ensure this behavior, and the method will not error-out, test
coverage is added for AwardEmoji#url.

Changelog: fixed