Completing #254 - cleanup splitting "shared" and "utils"

Final cleanup for #254:
* Move utils/topic_tools.py to shared/topic_tools.py
* Move scripts for i18n and khan-exercisees out of utils and into i18n and khanload, respectively
* Move app-specific scripts for downloading subtitles into main, and moved utils/subtitles/subtitles_utils.py to utils/subtitles.py

Not much to review here; any comments would most likely be addressed by further reorganization.  Better to get this merged (avoid any future merge conflicts), and start banging away on it.

Since it's the weekend, let's wait until EOD monday (48 hours)

@dylanjbarth take a look?