Added mixpanel for message tracking

[ghost]

• Added 3 functions to ngDialog controller for Mixpanel tracking
• Using "Message sent" field to track global messages sent by anyone
• In addition using field specific tags to track messages sent for each area
  • "Around you message sent"
  • "Available housing message sent"
  • "Looking for roommates message sent"
  • "Message sent from main message view"
• For dashboard controller using 3 different templates to track 3 different mixpanel functions