Allow a wider range of characters in macro names, including emoji

Summary:
Fixes T6121. See PHI357.

• Allow emoji and other unicode (like Chinese characters) as long as you have at least three of them.
• Disallow macros with only latin symbols. These were previously allowed.

Test Plan: Created a macro for "🐶🐶🐶", then used it in a comment.

Maniphest Tasks: T6121

Differential Revision: https://secure.phabricator.com/D19051