Add aliases for "party" emoji (🎉)

Summary:
This is currently :tada:, which I'd never have guessed.

(This isn't a super scalable approach, but this emoji is in particularly common use. See also T12644.)

Test Plan: Typed :party, :confet, etc.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18993