Allow references to labels and milestones to contain emoji

What does this MR do?

If we put the emoji filter before the reference filters (which is the current state), each emoji will have a wrapper element that prevents the reference filter from detecting the presence of the emoji.

As this MR makes the emoji filter run 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 :100:, 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.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/40728, https://gitlab.com/gitlab-org/gitlab-ce/issues/53756, and https://gitlab.com/gitlab-org/gitlab-ce/issues/37762, if it's acceptable at all that is.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.