User Manual, contextual errors and handler refactor

Highlights

• autojudge now has a user manual, where people willing to use autojudge are guided with respect to basic functionality provided by the software.
• handler.py was refactored to make functions less try-catchy.
• Errors thrown on forms now pinpoint the exact fields causing the errors, leading to better localization.
• superusers created via Django now are registered now have a dummy @autojudge.superuser domain.
• add_person_rgx_to_contest is removed from handler.py.
• submission_watcher_saver.py doesn't run in a spinlock-y manner if number of submissions are lower than a threshold, it instead sleeps for a while.
• Miscellaneous documentation improvements.