Don't bring up multiple help dialogs when the user hits "?" multiple times

Summary:
Some day, maybe close the existing dialog too but there's no public method on
JX.Workflow for that right now.

Test Plan:
Hit "??????", then "esc", got back to the page instead of just popping a deep
stack.

Reviewed By: tomo
Reviewers: tomo
CC: aran, tomo
Differential Revision: 450