Additional backend validation

[ZhangYiJiang]

Meta issue to track any problems with the backend validation - post any problems with the validation here as comments

• Reject duplicate
  • [x] Track names
  • [x] Venue names
  • [x] Category names
  • [x] Speaker names
  • [ ] Tag names
  • Duplicate sessions are not rejected because of Lunch
• [ ] Sessions should not be allowed to be created outside of the duration of the event itself
• [ ] Subject lines (title of sessions, event, names, etc.) should not contain new line character (to prevent email header injection attacks)