Tutoring session chat whiteboard prevent clearing

[sumanyu]

List of changes

• Used event emitter to pass messages back and forth between the tutor and tutee (clicking accept, start session, etc.)
• Added several before, after hooks to ensure all data is loaded before loading a template
• Added tutoringSession schema
• Organized how we initialize session variables, subscriptions and autoruns in main.coffee
• Chat is permanently stored on our servers so refreshing on tutoring session doesn't clear chat; it does clear blackboard
• When you create a question, the server now returns the questionId
• Removed all events from template questionTileLeft because we want to catch events for the specific sub-templates of that template
• Moved populateQuestions and other aux fns to fixtures.coffee because main was becoming cluttered
• Added better security to LineStream

  To do

• Save blackboard state so refreshing restores blackboard state
• Add better security to ClassroomStream
• Use username instead of id in chat
• Fix eraser issue when remote person erases but local user sees draw action
• Add undo action to blackboard
• Pay attention to what information user has access to for local collections; we'll probably need to tighten the security later