Remarkup - add as much image dimension hinting as possible

Summary: this was done for conpherence so the auto-scroll actually works. NOTE we actually use the 220 preview UI for file attachments right now so this really only helps in the macro case. :/

Test Plan: sent some conpherences with macros and files. verified image width / height was set as expected.

Reviewers: epriestley, chad

Reviewed By: chad

CC: aran, Korvin

Maniphest Tasks: T2399

Differential Revision: https://secure.phabricator.com/D4678