Timed exam bug fixes

[flysaiah]

Various bug fixes appearing in timed exams:

1. History slider used to disappear when cycling through questions because its functionality wasn't re-initialized.
2. Because of the new re-initialization function, it was possible for some buttons/etc. to have their listeners doubly registered, which caused weird behavior (for example two sliders would appear when clicking "Load history" in activecode).
3. When a drag and drop component was the first in a timed exam, nothing would be rendered because the timed exam would try to grab the DND's container div before it was finished rendering. In other words, it a component took too long to initialize, nothing would render in the timed exam. Rather than implementing a busy wait, this was fixed by rendering the question when the user clicks "start" the exam rather than when the page is loaded (unless we're loading from storage).