Summary:
Fixes T12460. Also ":)", ":(", ":/", and oldschool ":-)" variants.
Not included are variants with actual letters (`:D`, `:O`, `:P`) and obscure variants (`:^)`, `:*)`).
Test Plan: Typed `:3` (no emoji summoned). Typed `:dog3` (emoji summoned). Typed `@3` (user autocomplete summoned).
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T12460
Differential Revision: https://secure.phabricator.com/D17577