If we put the emoji filter before the reference filters, each emoji will
have a wrapper element that prevents the reference filter from detecting
the presence of the emoji.

As the emoji filter now runs after the reference filters, references
must contain a literal emoji, not the GitLab Flavored Markdown
versions (:100`, for example).

A weird side-effect is that if you have a label with the 100 emoji, and
a label named 💯, then trying to reference the latter will work (link
to the correct label), but will render with the 100 emoji. I'm
comfortable with that edge case, I think.