Pholio - make inline comment box appear correctly

Summary: Fixes T3509. Generally tried to make the code more consistently use get_image_scale, as well as make get_image_scale aware that sometimes images need to be scaled because they're too tall (as well as too wide).

Test Plan: used the file from T3509. noted comment box appearing correctly as clicked, or clicked and dragged. submitted some comments and noted the reticles moved / scaled correctly as I resized the browser window

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3509

Differential Revision: https://secure.phabricator.com/D6418