Recompute statistics every time a quiz completion is provided

[danielemery]

Currently quiz statistics are computed on demand and then cached. This means that:

a) there is a significant cold start when a user asks for statistics and they result is not already cached
b) if a user completes a quiz and looks at statistics they will not see the results of their just-completed quiz

As an alternative it would be good to rerun the computation every time a quiz is completed.

• This should not be done on the submit completion request but rather just started there (ideally later this will be on a worker but we don''t have one yet).
• This should also be done on application start so the cache is already hot
• The cache expiry should be removed or made very long (as it's now invalidation on completion it should not get out of date)