Detect macros more strictly in bot handler

Summary: Fixes T5863. Like Remarkup, only identify macros if the entire chat line is the macro.

Test Plan: https://secure.phabricator.com/chatlog/channel/6/?at=158229

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5863

Differential Revision: https://secure.phabricator.com/D10242