Make start_time fully timezone UNaware, ...

as our entire codebase assumes UNaware datetimes, and django requests consistency from us :)

Merging the recent video-update-update branch wind up breaking ... anything that stores datetimes in the DB (for example, UserLog functions).  `USE_TZ=True` can't simply be turned on, because datetime.datetime.now() is datetime UNaware, and we use it all over.

Quick fix was to set `USE_TZ=False`, which I did this morning--but of course, that broke the update to the video update code.

This PR fixes updates to use TZ unaware datetimes.

@jamalex will be testing this tomorrow, and the functionality is broken without this PR, so will merge straightaway.