Quest statistics

An attempt to #364 issue. It creates a QuestResult entry for users who started and didn't finish the quest. This will help computing the statistics for each level and it makes a distinction between users who started the quest and those who actually completed it.

It has something that doesn't work as it should, though. Until you press the "Register finished quests results" button, in the quest history page it shows that the number of players who attempted the quest is equal to the number of players who completed it (because these numbers are computed from the QuestResults entries, and the entries are created upon pressing the button).