Notify user actions on chat

[sumanyu]

List of changes

• Added load and unload hooks to classroom session. I was using them to test user entering and leaving the page, but the solutions were complicated and unreliable across browsers. I'm keeping the hooks for now.
• I moved logic to check if user has a valid classroom session in a before hook from action
• Abstracted alerting functionality in alertClassroomSession. We call this generic alerting function from starting and ending sessions and uploading files.

  Do it later

• My experiment to detect users abruptly leaving failed. We'll pick it up later if it's absolutely necessary.

[davinwong]

'enter' makes sense to me. good to go.