remove ProblemProxyAdmin
return empty string if to-be-hashed string is empty
do not create QuizBlockProxy and ProblemProxy
remove QuizBlockProxy, restore ProblemProxy to its original structure,
modify UserAnswer to be able to link to LabProxy and Problem directly.
remove unused management command
save hashed_sentence
management to migrate UserAnswer data
more UserAnswer admin display
unicode for ProblemProxy
check for hashed first then quiz_id
use quiz_id from problem first
method to get problem from quiz_id or message
refactor answer API, user problem and remove post to xblock