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