Allow disabling JX.Workflow with keyboard modifier

Summary:
Ctrl+click opens the link to a new tab in most browsers.
Shift+click to a new window.
Alt+click or Meta+click downloads the target.

This diff respects these conventions by disabling JX.Workflow for these modifiers.

Test Plan:
Click Flag Task - inline dialog.
Ctrl+click Flag Task - new tab with standalone dialog.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2276