Small changes to support worked example

Test Plan: Did a lot of work with these changes, and found that there
are reliably no problems, and the show worked example event regularly
gets triggered.

In particular:
    With the most recent version of khan-exercises checked out
       preview show_worked_example as an example
       click on the worked example button
       verify a worked example shows up

Auditors: jack, tom
Merge changes for worked example into khan-exercises