Added random session pair choosing

Due to a bug I cannot test this immediately
Put back the sort for the leaderboard handler.
Made editing thangs in the level editor not so slow by avoiding calling level.serialize.
Merge branch 'master' of https://github.com/codecombat/codecombat
Fixed random sessions fetching
Removed defaults on schema for ladder-related things
Hotfix for #550
Another hot fix for #550
Fixed bug in next game button, fixed wizard positions
Working on new loading view design.
Fixed two bugs with destroying sprites.
More work on the loading view.
Merge branch 'loading-view'
Oops, don't keep sending ladder updates just to me.
Fixes issue where panning occurs upon mouse scroll at max/min zoom
Enables drag-panning for editor and region chooser
Clicking a thang in the left list automatically pans to the sprite on the editor map
Non-selectable elements are no longer selected when corresponding thang is clicked in thang list in editor
Restricts wizard to level bounds when moving around
Merge remote-tracking branch 'origin/master'