Don't summon the emoji autocompleter for ":3"

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