Use file.download to retrieve macro images in the IRC macro bot
Summary: Fixes T5884. Macro images are no longer public on most installs. We could generate tokens for them, but this (using Conduit to pull the file data) is easier and more correct.
Test Plan: Logged a bot into IRC and had it spam part of a macro before being killed for flooding.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T5884
Differential Revision: https://secure.phabricator.com/D10274