Defer loading of flat_topic_tree until the first keypress.

Issue:
Pageloads are still quite laggy after the search checkin.  This is due to the client parsing a 1.1MB json file on every request.

Changes:
For now, defer getting/parsing of the json until the first keypress.  The lag for users (hitting the 3rd key) should not be noticeable, except perhaps the first time the json is retrieved.

PR help:
* @aronasorman, what do you think?