Move Macro image height/width to CSS

Summary: This makes macros and memes grow to 100% of their container at most, instead of showing a scrollbar. This is useful for overly large macros, smaller spaces like Feed and Conpherences, and Inline Comments. Fixes T7528

Test Plan: Tested a very large macro, a very large meme, and a very very tiny macro. It looks like memes get cached though, unsure if we should clean them up or just leave them

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7528

Differential Revision: https://secure.phabricator.com/D12045