Don't allow the middle mouse button to start an inline comment

Summary:
Ref T13216. See PHI985. When you click a line number to start an inline comment, we intend to initiate the action only if you used the left mouse button (desktop) or a touch (tablet/device).

We currently have a not right condition for doing this, but it only excludes right clicks, not middle clicks (or other nth-button clicks). The not right condition was sligthly easier to write, but use an is left condition instead of a not right condition.

Test Plan:

• In Safari, Firefox and Chrome:
  • Used left click to start an inline.
  • Used middle click to do nothing (previously: started an inline).

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13216

Differential Revision: https://secure.phabricator.com/D19836