Resolve great internal confusion about left vs right inline comments

Summary:
This code was just all kinds of wrong, but got all the common cases anyone cares
about correct.

• In edit-inline-comments.js, if isOnRight() is true, use data.right, not

data.left (derp).

• Set data.left correctly, not to the same value as data.right (derp derp).
• Set "isNewFile" based on $is_new, not $on_right (derp derp derp).

Test Plan:

• Added JS debugging code to print "OLD" vs "NEW" and "LEFT" vs "RIGHT".

Clicked the left and right sides of diff-vs-base and diff-vs-diff diffs,
verified output was accurate in all cases.

• Added comments to the left-display-side of a diff-of-diffs, saved them, they

showed up where I put them.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T543

Differential Revision: https://secure.phabricator.com/D1567