lose help cursor on blur

Summary:
Fixes T8501.
When losing focus while holding ctrl, we never get a key-up event; ctrl-f/d/tab make the browser tab lose focus.
So treat 'blur' (unfocus) as if the user released ctrl.

Test Plan: ctrl-f/ctrl-d/ctrl-tab, ctrl-click-outside-of-window, and move mouse over the content - see no help suggestions.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Maniphest Tasks: T8501

Differential Revision: https://secure.phabricator.com/D13260