Don't show solution log if task is not closed
Add the sidekiq role to capistrano
Add a checked column to ChallengeSolution
Add more photo sizes

The new sizes are: 150x150, 80x80, 50x50 and 30x30. Not all are used,
but probably all would be useful.

Another important change is that all of them are now named consistently,
thus no need to select the user in order to predict the filename of
their photo. This is a breaking change and it requires regenerating all
versions. This can be done with:

    rake users:photos:recreate

This should be executed after the code is deployed.
Improve the challenge interface a bit
Make the challenge test case editable
Extract checking queued jobs to a module
Generalize QueueMonitoring to work with any task
Fix ChallengeSolution.correct?

It should not return nil, since ActiveRecord tries to put null
in the database
Proper visibility of challenge solution results
Allow checking challenges
Remove N+1 queries on challenges#show
Add a hook for recompiling the lectures